Uhaul Refunded A Large Portion Of My Moving Costs For Some Reason

Hello everyone. Almost 2 years ago, I paid for U-Haul to move my stuff from California to Missouri. Everything went great. 3 weeks after the completion of the move, they refunded over $3,000 of the total moving cost back to my credit card.

After 8 months of going back and forth with them on the phone, and them opening an investigation into the matter twice, they still had no explanation how it happened, and refused to take the payment back. Supposedly because their system showed that I was paid in full. Even though, they did have record of them refunding me the money.

The last person I spoke to advised me to just hang on to the money, and that their system would eventually work things out on its own. That was almost two years ago.

Every first of the month, I would log into my U-Haul account just to make sure that I didn't have a balance due. Never once did I have a balance due. Until the 1st of this month. Then it showed that I had a balance due of the exact amount that was refunded to my credit card almost two years ago. But, when I press the button to make a payment, the next page it took me to said everything was paid in full, and gave no option to make a payment.

I called U-Haul, and they said they had record of the problem, but couldn't explain how it happened, nor could they accept a payment, because my account still said it was paid in full. They then said they would open an investigation into the matter, and should get back to me within 3 days. That was a month ago.

I've had the money sitting in a high-yield savings account since it was refunded to me. My question is, at what point can they no longer ask for their money back? I've tried every way I could to pay them, to no avail.