How can the amount due be calculated in Euros or US Dollars to settle a debt in German Marks, taking into account the change in exchange rates and the abolition of the Mark?

If the borrowed currency becomes unavailable, you must repay its value at the occurrence of the latter of two events: the time it became unavailable, or the time its repayment is due. If the debt is deferred, its value is considered at the time the debt becomes due. If you repay the debt now, the value of the mark is calculated at the time of repayment, but if the debt is due immediately and you are able to pay it, its value is calculated at the time it became unavailable.