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 been converted to passbooks, which should open up some space in the bags. I admit, though, I didn't think that he'd attach wires to the zippers; that's very smart of him.