We sort of work like that, we'll save up or borrow from our emergency fund, but yeah we try to stay equal with things - like if he's going on a boys weekend, we agree that he arranges for the boys to stay with his parents for some of that time.
It's sort of agreed here, as opposed to retaliation, but it may as well be, I mean, if he's just spent money on something and I call and check, but remind him of the money he's just spent - what else can he say but 'yeah that sounds fine'?