Parties compelled to testify.

When in an action to recover a debt the defendant claims that payment of unlawful interest on the debt has been made to the plaintiff or those under whom he claims, which the defendant is entitled to have credited on the principal of the debt, the plaintiff or the party who received the unlawful interest may be examined as a witness to prove the payment, and may not be excused from testifying in relation thereto. A creditor who is made defendant in a proceeding for discovery as to payments of unlawful interest made to him may not be excused from answering.