Just ask him how much a hour he charges for staying open. I ran into this last year playing at a room I had never played at and he kept the place open a extra couple of hours and I ended up winning 300, after getting loser 500, I was in a bad game and outran the nutts, and guess what he charged me? Yep, 300. After word got around it cost him about 5 regulars that knew me. In that city you can't have beer and pool tables as a business so, they got around it by not charging for the pool and people just gave a donation in a jar when they played. I will always ask from now on how much they charge per hour for staying open.
It sounds like you have something you don't want to mess up, but I would still talk to him and get a exact figure on what he charges. He showed himself by taking out the 100 like he did and you don't want to make a super score and him take out a percentage on you. You also want to always be on the good side of the owner.