The so-called 'exit bill' if it gets paid (no deal and it won't be) is according to the EU simply 'settling the accounts', it is money we apparently already owe and would have owed even had we remained. So we are paying less than we would have.
Secondly the forecast drop in finances hasn't happened. In fact rather than public finances deteriorating they are actually improving considerably better than forecast. He says funding is forecast to be £15bn worse off except as I posted the other day and he doesn't seem to have caught up on since those projections were made the UK has beaten the 2016 borrowing forecast by £22bn and beaten the 2017 borrowing forecast by £18bn.
http://theworldforgotten.com/showthread.php?t=6900
The people who wrote up Project Fear and got it all wrong forecast a catastrophic deterioration in finances already and they got that wrong too. Finances are going better because Brexit is going so well so far.