Assignment

Scenario:

Kevin signs a contract to sell his house to Joe.  However, Alex, without knowing about the deal with Joe, then offers Kevin more money for the house.  Since Kevin is getting married soon and needs the extra money he decides to sell the house to Alex, but does not tell him about the existing deal with Joe.

Questions:

1.     Does Joe have a valid claim to sue Kevin?

2.     If Joe has a valid claim, what are Joe's remedies?  

3..     Does Kevin have any defenses?