Apple ID selling issue

Friends, I must have done some trouble

I sold an Iphone 8 plus of my property

I have transferred the data to the new Iphone 13 purchased

so i kept my apple id and password.

On the old Plus 8, I did a reset

on I cloud / Itunes I have no longer associated this PLUS 8 between my devices.

Now the new buyer tells me that the IPHONE8 is blocked, that is, he cannot enter his APPLE ID, as if it is still associated with mine, so you cannot log in .... but I cannot now send him my apple ID and password to unlock it!

the phone sold is now distant ... and therefore I know that only APPLE can unlock it, deactivating my old APPLE ID from the IMEI ... or am I wrong? obviously I have all the necessary data.

SOS please.

JIMI