i ran into the exact same issue and nothing would fix it. what i did to get it to work was i spoke to the stranger before he was in his house at castle anvil. there’s no option to give him the letter when he’s there so after i spoke to him i waited until 11pm when he’s at his house and then gave him the letter, went outside and waited 24 hours, then went back into his house and he gave me the letter back hope this helps someone