There are two subtly distinct arguments anyway, and the conclusion for both is sell him.
Either he intends to leave at the end of his contract, and the destination is irrelevant. In which case Gyokeres showed a year left is no barrier to a fat fee if a player has enough admirers, and there'll be enough interested clubs that one will be if interest to the player. Shame it couldn't have been done quicker and cleaner like Gyokeres, but taking the cash is only right.
Or, he's homesick and therefore limits a transfer to a limited number of not overly wealthy clubs. If it's for personal reasons that risks affecting his performances, and it really isn't the ethical thing to do to force a player to stay for a year longer when there are valid mental health reasons why he needs to move. Therefore, the only conclusion must be to sell, accept the fee is substantially lower, but insert a nice fat sell on fee in case those personal issues resolve.
The wildcard option would be to agree to loan him to a Dutch club with an option to buy maybe, in exchange for him signing an extension.