Is this an enforceable contract? Why or why not?

Serena Maloney has married Ronaldo Espinoza, European royalty. Serena convinced Ronaldo that a prenuptial agreement was not necessary. Two days after the marriage Ronaldo has second thoughts and wants to have an agreement especially since his family not only has property in the United States but has villas in Spain that have been in the family for hundreds of years. Your attorney has requested that you (1) research the type of agreement that should be drafted and its contents and (2) determine what the laws in your jurisdiction are regarding post-marriage real property agreements.

Mr. Brown agrees to sell Mr. Blackwell the house at 423 Cherrytree Lane for $55,000. Th ey prepare the following document on a piece of paper: I, Mr. Brown, agree to sell my house to Mr. Blackwell. Mr. Brown (signed) Is this an enforceable contract? Why or why not?