REAL ESTATE LAW

Billy entered into an oral agreement to sell a parcel of real estate to Reggie for $20,000. Reggie paid Billy $500 as earnest money and Billy signed and gave Reggie the following receipt: "For $20,000.1 agree to sell the property at 120 Oak Lane. Received as earnest money $500 to Billy Jones" A month later, Reggie offered the remaining $19,500 to Billy but he refused to convey the property. What arguments, if any, should Billy raise in claiming the contract is not enforceable?