Vehicle sold – “false” vehicle letter

Haloo together, I have a really complicated problem. And this is what I sold my old car this weekend. When the buyer wanted to let the car today, he was told that the car letter he has “devalued” and that there is a new car letter to the car. But I don’t know anything about this one. I bought the car then as a young used car from an Audi dealer. The buyer can’t let the car now. How does he get to se right? He has a contract as well as the car bill, but I am still in it as a holder. What can he (or I) do now that he can allow the car? I think the easiest way would be to report the loss of the letter and to apply for a replacement letter. But who has to do this? The buyer is already the owner of the car right now, but not the holder. Thus neither he nor I can actually apply for this replacement? What other possibilities? Would there be? Need really urgent help, as I don’t want to rewind the sale at any price. I just have too much to do and no time.