It's definitely not money, since Fukumoto never showed the contents of the bag to us. Kaiji probably put something special in the bag to distract them and put the rest of the money in the other bags, since it's natural that the bags aren't stuffed to the brim. After all, some of the cash has...
@hyperactivehedgehog I'm fairly convinced that the person who threw boba/bubble tea at Rentarou is a potential reverse trap. Personally, I'd like to see his childhood friend from Chapter 1 end up being one, although that would mean that Rentarou never made eye contact with him for the past 39...