Is it permissible to receive interest on a loan made to a convert if the loan agreement was made before the conversion and repaid after?
Rabbi Yisroel Reisman writes in The Laws of Ribbis:
If a Jews lends money to a non-Jew, and the non-Jewish borrower then converts to Judaism, the lender may not collect interest which accrues after the Geirus [=conversion to Judaism]. However, interest which accrued before the Geirus must be paid. In this case, there is no difference if the bill (or promissory note) was written before or after the borrower had become a Ger [=convert to Judaism].
He cites this from Yore Dea 171:1. He adds in a footnote:
Shaar Deah (171:2) rules that the lender may demand that the loan be repaid immediately. Since he made the loan on the assumption that he would be paid interest, and he is no longer able to do so, he may retract his commitment.
See Bris Yehudah 30:7 regarding a situation where no bill or promissory note was written at all.
It seems me'ikar ha'din this would be permissible because the issur of ribbis relates to the mitzah to re-pay the loan (not the chiyuv of mamoni gabach because then there would be an issur to take ribbis from an akum) and there can be no mitzvah to re-pay this loan because a Yisrael never undertook payment thereof.
However mitzad the taam ha'mitzvah it would seem it is assur to accept the ribis because it is still included in the issur of es kaspo nasan be'neshech chayo lo yicheye.
I.e. it would be permissible for the convert to pay the interest but it would be forbidden for the Jew to accept it.