What happens if a Jew borrows money on interest from a non-Jew, and then the lender turns around and sells the debt to a Jew? (Such transactions are very common in the mortgage industry.) Is it prohibited to pay interest to the new owner of the loan, since he is Jewish? If so, does the borrower need to pay up the loan immediately? What about any interest that has already accrued before he does so?
(For the purposes of this question, let's assume that we are dealing with individuals, not corporations.)