Actually, there were two money tricks. The first one is performed at the shop where Moses is buying some ribbons for Addie. Addie performs it again at the carnival when she buys cotton candy. The trick goes like this. First, you buy something trivial, e.g., the ribbons for 15 cents, then you pay for it with a $5 bill. The clerk gives back the appropriate change ($4.85). You pocket the 85 cents, add another $1 bill to the remaining $4.00 and ask to exchange it for the $5 bill you just gave her. So far, an equal transaction. While the clerk still has the five $1 bills in her hand, you add to it the $5 bill that the clerk just gave you and ask for a $10 bill back for the five $1s + $5 you just gave her. Do the math (or, better yet, try it with real money), and you'll find that you come up $4.85 ahead.
The second trick involves Moses going into a store and buying a tube of Ipana toothpaste and a pack of SenSens. He pays for it with $20, gets appropriate change, then leaves the store. Okay so far. Then Addie goes to the counter to purchase a bottle of perfume for 25 cents. She pays for it with a $5 bill and gets appropriate change ($4.75). Again, an equal transaction so far. As Addie prepares to leave the shop, however, she turns back and informs the clerk that she paid with a $20 bill. When the clerk balks, Addie begins to cry. The manager comes over, and Addie informs him that she paid with a $20 bill that was a birthday gift from her Aunt Helen in Wichita. She can prove it because Aunt Helen wrote "Happy Birthday Addie" on the face of the bill. The manager looks through the till and, sure enough, there is a $20 with "Happy Birthday Addie" written on it. He gives the $20 back to Addie, along with some candy. Addie leaves the store with $24.75 in her pocket, along with the perfume and the candy. And we all know who wrote "Happy Birthday Addie" on the $20 bill.