Jenny (one of Johnny's waiters) owes Johnny $500. Jenny tells Johnny that because of

unforeseen medical expenses she will be unable to repay the debt in full. Johnny agrees

to accept $300 in full satisfaction of the debt. The next day Johnny sees Jenny buying a

new ePod, and Johnny tells Jenny that since she is clearly not in that much financial

difficulty, Johnny has changed his mind and wants the debt repaid in full.

Can Johnny enforce the debt of $500?