Newsflash: Closed almost 3 years ago – Buyer demanding we pay for a Repair

This may be long winded….We sold our house almost 3 years ago. In the closing agreement, we needed a door replaced. The door was a custom order that was a few months out due to supply constraints. We ordered the door (almost $2k for the door) and had it shipped to our old house. There was an issue with the door supplier and the installer and they had a falling out and the door was slightly the wrong size. The new owner then wanted the door out of the house and it was shipped back to the supplier (at our cost).I then found a new installer who said he could install the door from the supplier, and the new owner did not respond after two attempts. Several months later, the new owner texted saying they wanted it resolved, we were overseas and asked the new owner to email us, but they never did.Now, after almost 3 years, we received a letter from an attorney saying they want it replaced and will sue us if we do not pay for a new door and install (the door is probably long gone which we are already out almost $2k for).Any advice here? I feel like we did our due diligence but they never responded when we tried to get the door installed. There was no money left in escrow for the repair, the closing documents just said it would be repaired. We tried to resolve it years ago on our end, but had no response.