Interest is just a function of a loan agreement - it is an entirely separate issue from the available supply of the currency in question.

Even if you borrow monopoly money from a specific monopoly set, there can still be interest required on the loan. It may be impossible for you to fulfil your loan obligations if your supply of whatever currencies the lender will accept (which does not necessarily only have to be the original loan currency) is limited, but that's a separate issue.