Responsa על בבא מציעא 150:8

Q. A claims that B lent him money on a pledge, and that when he came to redeem the pledge B exacted interest from him. Since he had contracted the loan directly from B, the latter was not allowed to take any interest on this loan. A, therefore, demands that B pay him back the interest. B, on the other hand, claims that the loan was originally contracted through a Gentile intermediary and that he was entitled, therefore, to the interest.
A. It is assumed that a person who might have gained an advantage by following a certain legal procedure, did not fail to follow such procedure. Since B could legally have been entitled to interest on his money had he granted the loan through a Gentile intermediary, we assume that he did grant the loan through such intermediary. Therefore, B is entitled to his interest and is not required to take an oath.
SOURCES: L. 197.