Most bars don't go for contracts. I make sure that I send an email to them confirming the date and the amount. I print it. When I get to the bar, I remind whoever the manager is using the email to show the amount. If the manager disagrees or wants to change the amount, I remind him of the email. When he tells me to take it up with the owner, he is only going to pay $X, I then talk to my guns for hire. If we agree to stay and play, we usually cut the sets really short and often cut a complete set on top of it.
I did this a year ago at a local pub who the owner was great but he hired a manager to run the place. The manager was a prick. When I did my normal check in routine, he told me that instead of the $150 per man (4 of us) he was only "authorized" to give us $375 total. I showed him the email, and he said he didn't care, all he was going to give us was $375. Instead of me calling the owner, we played a three song first set.
The bar manager came flying over and screamed at me. I told him, politely, that any lawyer and court would tell him that by him telling me he was only going to pay what he was, he constructively changed the contract, or in a sense voided it, so I was free to change any agreement that I had made as well. That meant that I could play as long or short as I wanted. I told him I would honor the three sets, however the length of each set was up to me.
Then he said he wasn't going to pay at all. We started to pack up, and asked our fans to leave the bar and just go wait by their cars. The bar was packed because we were celebrating one of my bandmates birthdays, maybe about 80 people in a bar that on most good nights might have 35 or 40 in at any one time.
The bar suddenly emptied, and I mean only two regulars left, and it was about 10pm. The managers face was white as a ghost. He came over to me and started crying about how it was going to be a big losing night now. I reminded him he said he wasn't going to pay, and had originally changed the amount we already agreed on with the owner. He asked me what he could do to get all those people back. I told him, "Don't screw the band." We left.
The owner called me the next day, was a little irate but appologized, and asked if I would play another date. I told him no, because the manager is an extension of you, and therefore you screwed me.
The bar closed for good about 6 months ago....