Clearly, one is not permitted to charge interest on a money loan. However, I believe there is no problem paying interest according to a previous agreement if the lender is not a Jewish person. And certainly one should not accept a loan from a Jewish person if the loan involves interest.
But it could easily happen nowadays, that the borrower did not know that the lender was Jewis, and then discovered that fact before the loan is paid back. Assuming that the borrower has no other options and even if the lender did not care that he would break a Torah mitzvah, is still one required to pay the interest?
Or should one not pay the interest and accept what ever financial consequences may result?