How do I get my deposit back?

I put a 600$ deposit down before the 1st of the month for a house with caldwell banker on memorial day weekend. I called back tuesday saying that I didn't want it anymore. I got my deposit back for 300$ today with the lady telling me she really didn't have to give any of my money back. She says I took her house off the market and is losing 300$ because of it and could stick me out in the cold with 600$. How do I get my full deposit back. I didn't sign ANYTHING for this house NOTHING NADA ZIP ZERO, I have her receipt and her letter with the deposit both signed by her. HOW DO I GET MY MONEY BACK I NEED IT FOR SCHOOL I don't know what to do this was hard worked for money that I could have just kept and now its gone please someone tell me that there is a way for this to be flipped.