Farmer owed Johnny $1000 for a loan that was due and payable. Farmer found himself short of cash and was unable to pay. Johnny offered to accept in full discharge of the debt a certain Guernsey cow, which Farmer had on the farm. The cow was transferred to bobby and a signed agreement was executed by both parties setting forth the deal. Two months later Johnny seeks advice as to his rights against Farmer for the remaining $400 claiming the cow is worth only $600 on the market. Would Bobby be successful in his legal attempt to recover? Explain and illustrate the legal concepts and issues involved.