A national bank offering or purchasing ARM loans may impose fees for prepayments notwithstanding any State law limitations to the contrary. For purposes of this section, prepayments do not include:
(a) Payments that exceed the required payment amount to avoid or reduce negative amortization; or
(b) Principal payments, in excess of those necessary to retire the outstanding debt over the remaining loan term at the then-current interest rate, that are made in accordance with rules governing the determination of monthly payments contained in the loan documents.