So there are all sorts of questions involved: Is actual payment required? Under what conditions may you cancel the lease? Are there contingencies in the lease that you could invoke? What sort of landlord-tenant regulations might protect your would-be tenant (or, possibly, you)?
Check with a lawyer.
It's best for you to consult with a real-estate attorney. Depending upon the terms of your agreement, and how your potential tenant submitted that agreement to you, then it's entirely possible that your agreement might not be effective (or binding). Your attorney will determine that, and whether any other course of action is necessary.
GOOD LUCK and let us know how it works out.
Scott Miller, Realty Associates, Boca Raton, FL