Dusty's elderly mother asks him to go to Walmart to purchase only those groceries on her list, but tells Dusty to buy nothing else. She writes a check payable to Walmart, includes the date, and signs the check. However, because she does not know the precise cost of the groceries, she leaves the amount blank and tells Dusty to fill in the exact amount of the groceries. The cost of the groceries is only $45. When the cashier isn't looking, Dusty writes the check out for $65, because he knows Walmart has a policy of providing cash back on checks up to $20. The cashier sees only the properly completed check and provides him with $20 in cash back. Can Walmart enforce the check? Does Dusty's mother have any defenses? Why or why not? Short answer, answer the question to the fullest extent.