What do you do with the money you got back from the price of the shirt that you returned without your brother's knowledge, and which he had worn and washed before it was returned, and he refuses to take the money?

If the son is an adult and of sound mind, his gift (hibah) may not be disposed of except with his permission. If he is not an adult, his father takes possession of the gift and disposes of it in a way that benefits the son. If the son or his guardian has given permission to return the gift, there is no harm in that. If it becomes clear that the brother has used the shirt and washed it, the seller must be informed. If the seller accepts it, then so be it. If he does not accept it, then the shirt belongs to the son, unless the son permits its return to his maternal uncle.