Your a reasonable landlord, so they have taken advantage of your kindness. Send them an e-mail; tell them they can either pay it off, per your agreement, or you will take it off the deposit at the end of their tenancy. I'd give them a deadline of a week from the time you send them the e-mail to respond with option a or b; no response will be assumed to be option b. Word it in such a way that you are aware that they have breached both their lease and the agreement you made with the daughter and that it must be addressed now and rectified. Stand your ground and don't listen to any arguments. A contract is a contract.