
356 S.E.2d 805 (1987)
SAMPSON-BLADEN OIL CO., INC.
v.
Gerald WALTERS and Joyce Walters.
No. 8613DC893.
Court of Appeals of North Carolina.
June 16, 1987.
*807 Young, Moore, Henderson & Alvis by Edward B. Clark and David M. Duke, Raleigh, for plaintiff-appellant.
Lee, Meekins & Viets by Fred C. Meekins, Jr. and Junius B. Lee, III, Whiteville, for defendants-appellees.
PHILLIPS, Judge.
Other than a contention about the interest allowed on defendants' recovery, the assignments of error brought forward in plaintiff's brief support only these three contentions: (1) The order of summary judgment in plaintiff's favor the day before trial began disposed of the entire case, and thus deprived the court of jurisdiction to try defendants' counterclaim; (2) the court abused its discretion in permitting defendants to amend their counterclaim the day trial began; (3) the court erred in ruling as a matter of law that overcharging defendants under the circumstances recorded was an unfair or deceptive trade practice under G.S. 75-1, et seq. None of these contentions has merit and we overrule them. In doing so we will not discuss plaintiff's other contentions that are not duly supported by an exception or assignment of error, as such matters are not properly before us. It is appropriate to note that plaintiff's appellate counsel did not participate in either the trial or preparation of the case.
*808 So far as the record indicates the contention that the order of summary judgment entered the day before trial disposed of the entire case was first made in a post trial motion a week later. Nothing in the order requires that interpretation; for it does not mention defendants' counterclaim and merely recites that during 1983 defendants received the merchandise listed on plaintiff's verified statement and owed plaintiff $4,080.22 therefor. Neither the record, the transcript, nor the conduct of the parties and counsel indicates that the order was intended to dispose of the entire case, or that anyone connected with the case so thought or maintained when it was entered. Though the complaint, answer, counterclaim and reply raised several issues of fact only one of those issuesthe amount defendants owed plaintiff for oil supplied to them during 1983 raised by the complaintwas addressed by plaintiff's motion for summary judgment; and that motion was supported only by a verified statement of plaintiff's account with defendants for 1983. The verified statement merely lists the deliveries of oil made to defendants during 1983 and the charges made for them and does not mention defendants' verified counterclaim for a set off and treble damages based on plaintiff's overcharges during 1982. Since the court had before it only the pleadings and a verified statement showing that defendants owed plaintiff $4,080.22 for oil supplied them in 1983 it was proper to enter an order of summary judgment disposing of that issue; but the court had no basis for disposing of any other issue in the case and did not undertake to do so. The recorded facts indicate that the court, parties and trial counsel all regarded the order as being one for partial summary judgment that affected only the issue of defendants' debt for oil received during 1983; for almost immediately after the order was entered the trial of defendants' counterclaim was begun without any objection from the plaintiff. Only by regarding the order as being for partial summary judgment does the course followed by the court and counsel make sense; and only by regarding the order as being for partial summary judgment can the validity of both the order and judgment be upheld. But if the order is construed as attempting to dispose of the entire case it is a self-evident nullity, since plaintiff's evidence at the hearing did not even address, much less support, the dismissal of defendants' counterclaim.
Nor did the court err in permitting defendants to amend their counterclaim to include overcharges made in 1981. Though the motion to amend was not made until the trial was ready to begin, plaintiff was notified more than a year earlier when defendants answered the complaint that its 1981 charges were an important factor in the case. For at that time defendants alleged that because the 1981 charges appeared to be excessive they kept up with the 1982 deliveries and ascertained that plaintiff was overcharging them. Under the circumstances it seems unlikely that plaintiff was either surprised or prejudiced by the amendment. In any event allowing the motion to amend was within the broad discretion that Rule 15, N.C.Rules of Civil Procedure, gives to our trial judges and was certainly no abuse of it. Willow Mountain Corp. v. Parker, 37 N.C.App. 718, 247 S.E.2d 11, disc. rev. denied, 295 N.C. 738, 248 S.E.2d 867 (1978).
In discussing its contention that no unfair trade practice was established plaintiff argues, inter alia, that the evidence is not sufficient to support the jury's finding that plaintiff overcharged defendants. Since this contention is based upon assignments of error and exceptions that relate only to the court ruling as a matter of law that the overcharges the jury found plaintiff made constituted an unfair or deceptive trade practice under G.S. 75-1, et seq., the sufficiency of the evidence is not before us and will not be decided. Though plaintiff strenuously argues otherwise it seems plain to us, and we so hold, that systematically overcharging a customer for two years, as the jury found was done here in the amount of $2,795.30, is an unfair trade practice squarely within the purview of G.S. 75-1.1, as our Supreme Court has interpreted it in several cases, including Marshall *809 v. Miller, 302 N.C. 539, 276 S.E.2d 397 (1981).
Plaintiff's final contentionthat the court erred in awarding interest on defendants' recovery from 15 September 1982 until the date of the judgmenthas some merit, but not for the reason or to the extent argued. Plaintiff labels the interest allowed as "prejudgment" interest, and correctly argues that prejudgment interest is not allowable in this case because no statute authorizes it. The only statutory provisions authorizing prejudgment interest in recent years are those formerly contained in G.S. 24-5, which only applied to claims covered by liability insurance, and the provisions now contained in G.S. 24-5(b), which apply to compensatory damages in all actions other than contract, but do not apply to cases pending when the 1985 General Assembly enacted them, and this case has been pending since October, 1984. But it does not appear to us either that the interest awarded defendants was prejudgment interest, as that term is generally understood, or that it was awarded under either the old or new version of G.S. 24-5(b). Apparently the court allowed interest under the provisions now contained in G.S. 24-5(a) and did so on the premise that defendants' counterclaim is based on contract; for these provisions, which have been in our statutes since 1786, authorize interest from the date of the breach in actions based on contract, and interest was allowed here not from 1 November 1984 when defendants' counterclaim was filed, but from 15 September 1982 when defendants' last overpayment was made. In awarding interest under that statute the court acted correctly up to a point; for under G.S. 24-5(a) amounts due by contract normally draw interest at the legal rate if not otherwise provided, and defendants' claim for money they overpaid plaintiff is based upon an implied promise by plaintiff to refund the overcharge. 70 C.J.S. Payment Sec. 114 (1987); Allgood v. The Wilmington Savings & Trust Co., 242 N.C. 506, 88 S.E.2d 825 (1955). Despite the implication in G.S. 24-5(a) that the fact finder must determine the amount of interest due in actions based on contract, it is immaterial that the jury did not compute the interest due in this instance; for, as our Supreme Court has held in many cases, whenever a recovery is had for breach of contract and the amount of damages is ascertained either from the contract or from evidence relevant to the inquiry interest, which any clerk can compute, should be added thereto as a matter of law. Rose v. Vulcan Materials Co., 282 N.C. 643, 194 S.E.2d 521 (1973); Hunt v. Hunt, 261 N.C. 437, 135 S.E.2d 195 (1964); General Metals, Inc. v. Truitt Mfg. Co., 259 N.C. 709, 131 S.E.2d 360 (1963); Harris and Harris Construction Co. v. Crain and Denbo, Inc., 256 N.C. 110, 123 S.E.2d 590 (1962); Bond v. Pickett Cotton Mills, Inc., 166 N.C. 20, 81 S.E. 936 (1914). In this case a breach of implied contract occurred each time during 1981 and 1982 defendants overpaid plaintiff and the latter failed to immediately return the overpayment, and the amount of the overpayments having been ascertained from the evidence interest thereon immediately attached. But it attached only to the overcharges, the only money of defendants that plaintiff had the use of; it did not attach to the statutory penalty that was added to the overcharges. Allowing interest on the overpayments from the last breach until judgment was entered does not duplicate to any extent the recovery authorized by Chapter 75; as interest on the money plaintiff had the use of stopped when it was established that the Chapter had been violated.
In arriving at $5,465.94 as the amount finally due defendants from plaintiff the court followed this course: It trebled the $2,795.30 overcharges to $8,385.90, added interest on that amount at 8% from 15 September 1982 to the day judgment was entered ($2,320.24) for a gross recovery of $10,706.14; and then subtracted plaintiff's $4,080.22 recovery plus interest thereon of $1,159.98. The only error in this procedure was in allowing defendants interest at 8% on $8,385.90 for the period stated, rather than on $2,795.30. This error improperly increased defendants' net recovery by $1,551.03 and the judgment must be modified accordingly. To expedite matters, we *810 herewith modify the judgment in defendants' favor to provide for their recovery from plaintiff of $3,914.91, rather than $5,465.94, and direct the District Court to correct its records accordingly.
Affirmed and modified.
BECTON and JOHNSON, JJ., concur.
