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? Include in your answer a consideration of whether or not Jenny can rely upon the doctrine of promissory estoppel.