Try this little brain teaser?

A lady came into Bob’s shoe store and bought a $75 pair of shoes. She gave Bob a $100 note. Having no change, Bob went to Jan next door, exchanged the $100 note for smaller notes and gave the lady $25 change and the shoes. Later that day, Jan came into the shoe store saying that the $100 note was fake. Bob apologised and, having more money in the till, gave Jan $100 for the fake note. How much money has Bob lost over this?