I am doing contract law and I have to answer question using the following case study:

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.

I was wondering if the requirement:

"Both parties must intend that their agreement be legally enforceable"

was evident in this and if so how? What evidence is there?