As a law professor told me way back in school, "you can always sue anybody... question is can you win and will it be worth it?" So you may be able to sue him on a contract theory, and show what you gave up to come to the U.S. in reliance on his promise to get married. Unless your losses are extreme, like you left your job as Nicaragua's top t.v. anchorwoman or left medical school with a semester to go, I don't really see an adequate damages possibility. Also, I'm thinking a judge/jury would say that you assumed the risk because not all engagements or even marriages work out. But my colleagues are all correct in that under the terms of the fiancee visa, you will have to leave if you don't marry the petitioning fiance. Sorry.
You must depart from the US.
Legal disclaimer: The statement above is general in nature, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship.
I agree with my colleagues.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.