This isn't proper "form" but I don't think there will be any penalty besides losing your deposit. Unless you sign some kind of contract you are ok to back out...
I am in this same situation and I emailed the department and told them I was in a financial situation right now and needed to wait on another school to show me what they were offering financially. I said that I needed to make sure I made the right choice for me in my life right now. As long as you don't place a value on each school it doesn't hurt to ask for an extension.