12 CFR § 226.54 - Limitations on the imposition of finance charges.
(a) Limitations on imposing finance charges as a result of the loss of a grace period -
(1) General rule. Except as provided in paragraph (b) of this section, a card issuer must not impose finance charges as a result of the loss of a grace period on a credit card account under an open-end (not home-secured) consumer credit plan if those finance charges are based on:
(ii) Any portion of a balance subject to a grace period that was repaid prior to the expiration of the grace period.
(2) Definition of grace period. For purposes of paragraph (a)(1) of this section, “grace period” has the same meaning as in § 226.5(b)(2)(ii)(B)(3).
(b) Exceptions. Paragraph (a) of this section does not apply to:
(2) Adjustments to finance charges as a result of the return of a payment.