This is really not a probate question, but a simple one of accord and satisfaction. Any creditor can forgive a debt for any or no reason, and it appears aunt did that. If the administrator still insists on repayment after you have shown him the correspondence and card, you may then raise it as a defense in any case the administrator commences against you.
Anyone can be sued. The issue is whether the suit can be sustained. If you have a copy of your letter and the original card, you should be in good shape. Although it was a loan, it may be forgiven. It is not legally a gift. It is the foregiveness of a debt.
This response does not constitute the establishment of an attorney-client relationship. It is also not to be taken as firm legal advice as such would be contingent on a full inquiry by the attorney into the complete background of the facts and circumstances surrounding this matter. The response is meant to be a helpful guide to a question in a manner which reflects the limited information provided by the inquirer.