Hello and thank you for the opportunity to assist you. My name is ***** ***** I will do my very best to answer your legal questions.
The best way to deal with this type of situation is to get all of the terms in writing (including a specific closing date with a statement that "time is of the essence") and include a statement that these terms comprise the entire agreement, and that nothing shall modify this agreement unless it's in writing and signed by all parties.
If you already have something like that, then there is no reason for you to give any type of deposit at all if it's not already in the agreement. Of course, the difficulty is that the buyer may know that you have pressure because of the purchase of your new house in September, and that any delay will give you trouble. On the other hand, the buyer has pressure as well because he paid earnest money and doesn't want to lose it by refusing to settle.
But the bottom line is that the way to protect yourself is to get all terms (including important dates with a statement that "time is of the essence") in writing and signed by the buyer. If you have that, then any breach would allow you to keep the earnest money and search for a new buyer. Realistically, that won't happen because people don't want to lose their earnest money for nothing more than stubbornness or gamesmanship.
Does that answer your question? Please let me know if you need clarification, as I am happy to continue helping you until you are satisfied.