Please analyze the following problem. Be sure to state your reasoning.

Buyer receives in the mail from Owner a signed document that constitutes an offer to sell Owner's land. The offeror specifies "You must sign this offer to accept and when you have done so there is a contract between us. Please return this document to me after you sign it." Buyer signs the document but before he can place it in the mail Owner calls on the phone and states "I'm sorry but I have to revoke my offer." Later that same day, Buyer mails the document back to Owner. Is there a contract between the parties?