175.2.15. Where the amount, expressed in the weak currency, outstanding on account of principal in respect of a debt that is at any time a weak currency debt is reduced before maturity, whether by repayment or otherwise, the amount, expressed in the weak currency, of the reduction is deemed, except for the purpose of determining the rate of interest that would have been charged on an equivalent debt in the final currency and applying paragraph b of the definition of “weak currency debt” in section 175.2.12, to have been a separate debt from the commitment time.