Are you literally saying he should not be rational or am I misunderstanding something here? For simplicity let's say that his salary with bonuses is 100, his salary without bonuses is 80 and the salary he'll get elsewhere is 50.
His options in your scenario are these:
1a. He can stay here and sit on the bench, he'll get 80.
2a. We buy him out and don't pay the bonuses, he'll get 80 + 50 from his new club = 130.
3a. We buy him out and pay the bonuses, he'll get 100+50 from the new club = 150.
Our options are these:
1b. He can stay here and sit on the bench, we pay 80.
2b. We can buy him out and don't pay the bonuses, we pay 80.
3b. We can buy him out and pay the bonuses, we pay 100.
Do you see the logical outcome in this negotiation as him getting 100 from Arsenal or are you talking about a different situation than I am?