Ala. Code § 12-21-164

When the borrower is dead and usury is relied on as a defense, the representative of the borrower, having given 10 days’ notice to the plaintiff, or his attorney of his intention so to do, is a competent witness to prove the usury by swearing that he believes the contract to be usurious, if the plaintiff was the lender, unless the plaintiff denies on oath, in open court, the truth of the facts proposed to be sworn to by the defendant.