FNB Credit Card

My husband had a credit card with them almost 10 years ago, which was paid in 2012. Now, ten years later, when we want to buy a property it has come to light that there was still monies due. I tried to speak to the legal department, as well as a consultant, because we still bank with FNB, but to no avail. They told me that they don't have such a account on their books anymore, but I also don't want to provide me with a letter confirming that the account with FNB was closed.