I need more information.... did you sign a contract to sell your home to a buyer and then changed your mind? If that's the case, you are very lucky $300 is all you are being hit with. Did the buyer mutually agree to cancel the purchase contract?
I let sellers out of my listing contract free of charge if they aren't happy.... up until the time they sign an agreement to sell the home (purchase contract/accepted offer). At that point I fulfilled my end of the bargain and that is what I get paid for. If they want to cancel before we find a buyer.... then that's all good - and I'll help them next time around.
Another thought... have you just not found the right home? Is the same agent helping you with that side? If you had a strong offer, maybe you can extend out the time on the deal - to give you additional time to find a home?
Best of luck,