I say to my friend:
I told you I'd foot your bill if we get/got the guys to pay half.
Should I always backshift in this situation?
I would use got, but I don't think you don't have to backshift always- in colloquial speech if the deal hasn't been finalised, the present would work OK I think, but I don't think everyone would accept it.
It is a curious mixture.
I think "when" can replace "if".