Promissory estoppel. Have a local business lawyer handle it. Avvo has a terrific "find a lawyer" tool to find a top-rated Avvo attorney with a low contingency fee. Good luck.
Your statement of acts is confusing. Let me see if I can add some order to them (per my understanding):
1. The opposing party is not your spouse or anyone else's, correct?
2. You entered into a verbal agreement to help the opposing party with the opposing party's mortgage for an undisclosed amount of money in exchange to being added to the mortgage and being the opposing party's sole beneficiary in a will.
3. The house upon which the mortgage is predicated is now in foreclosure, and the opposing party is selling it in a "short-sale."
4. You want to know your options for recouping your money from the opposing party.
That is my understanding of your facts. The issue of the opposing party being an experienced real estate attorney is probably of little to no legal significance except for one facet: He would know that transactions involving the sale of land must be in a contract.
The Statute of Frauds controls here regarding all sales of land, transactions over $500, and contracts that cannot be completed within a year. You don't have a written contract, so that arguably gives the opposing party a strong defense.
However, you can argue promissory estoppel and state that it would be unfair to let the opposing party keep any unjust enrichment. The issue, though, is that the house would still be going into foreclosure, and in Florida, the courts are highly reluctant to force parties to add people to their wills. Probate is generally the preferred method of sorting out will contests.
I think your best bet is to sue for repayment of the debt under promissory estoppel and leave out all references to "spouse," but realize that the opposing party may have a very strong Statute of Frauds defense.
Keep in mind, too, that Florida abolished "common law marriages" in 1968, so no hope applies there. Legally, you are in the same classification as any other cohabitating couple. That may or may not assist you.
The larger issue is it sounds like the opposing party may be broke, so while the court may award you a judgment, it is up to you to collect on it. If the opposing party doesn't have assets, then you a stuck with a worthless piece of paper.
That, then, is my understanding of the issues. I don't practice in this area of law, but I had a truly excellent Contract Law professor, and I like the subject. Please do get a consultation with an attorney who does practice in this area of law, since s/he might have differing opinions.