How Do We Prove That We Paid Her?

My husband had to pay his ex $26,000.  He refinanced his house and cashed in part of his inheritance from his grandfather.  We are now just settling a custody battle for his youngest daughter who has been living with us since June.  When the lawyers got together they agreed that the daughter could stay with us and then suddenly his ex states that he owes her $17,000 from their divorce.  How do we prove that we paid her? – Lori