Slander is verbal defamation. Libel is written/printed defamation. Truth is a complete defense. (There is an exception for bad motive/malice which is too complicated to try to explain and is rarely an issue.)

It might not be the best thing to do though from a public relations standpoint, but if it is enough money, it might help you collect.

It is still possible to be sued. Also depending upon the nature of the transaction the Fair Credit Act might apply and that might be considered an illegal collection tactic. Do not deal with that myself. Perhaps someone else here can comment.