Responsa for Bava Metzia 143:6
תנו רבנן שטר שכתוב בו רבית קונסין אותו ואינו גובה לא את הקרן ולא את הרבית דברי רבי מאיר וחכמים אומרים גובה את הקרן ואינו גובה את הרבית במאי קמיפלגי רבי מאיר סבר קנסינן התירא משום איסורא ורבנן סברי לא קנסינן התירא משום איסורא
Similarly, if a heathen borrowed money on interest from an Israelite, and then recorded them [the principal and the interest] against him as a loan, and became a proselyte: if the settlement preceded his conversion, he [the Israelite] may exact both the principal and the interest; if it followed his conversion, he may exact the principal but not the interest. R. Jose ruled: If a heathen borrowed money from an Israelite on interest, then in both cases [whether conversion preceded the settlement or the reverse] he may collect both the principal and the interest. Raba said in the name of R. Hisda in the name of R. Huna: The <i>halachah</i> is as R. Jose. Raba said: What is the reason of R. Jose? That it should not be said that he turned a proselyte for the sake of money.<span class="x" onmousemove="('comment',' To evade the payment of interest. ');"><sup>6</sup></span>
Teshuvot Maharam
A. Your decision was correct and the trustee's stipulation of a definite interest was illegal and, therefore, void. However, under the circumstances, the loan is as if made by the Jewish court — the natural trustee and protector of all orphans — on the usual terms [made when orphans' money is loaned]: "to share in the profits but not in the losses." Therefore, if A earned profits with the money, he must pay to the trustee, the stipulated quarter per mark. If A admits that the money he borrowed belonged to the orphans, but claims to have repaid part of it, the trustee is believed as to the amount he received from A, (in repayment of the loan), and is not required to take an oath since the trustee is a disinterested third party. But if A states his belief that the trustee loaned him his own money, and not the orphans' money, the trustee must take an oath as to the amount he received from A in repayment of the loan, though he is not required to take an oath to the effect that the money he loaned belonged to the orphans.
SOURCES: Pr. 969; Mord. B.M. 332; Agudah B.M. 98.