Since the buyers backed out, and the contract states $250 will be held for the sellers attorney, then it seems that the seller's attorney should receive the $250 and that should be a full and final payment for a failed transaction that required only one hour of the seller's attorney's time.
Who referred you to this seller's attorney? A fee of $600 seems unreasonable for one hour of work. If the seller attorney is saying that they charge $350 for one hours work, then the contract owes him the $250 and you pay the other $100. In the mean time, I would find another attorney and get their fees in writing. Please let me know who the seller attorney/law firm is so that I do not fall victim. Thanks in advance.