Accept now and withdraw later?

International student here, pursuing masters in USA. I got into a safe school that is at the bottom of my list. Need to confirm my seat by depositing $500 by Feb 20th, whereas most colleges (including my dream schools) don't give out admits by March 15th. What should I do?

Can I secure my seat now and withdraw admission later (in case I get into my dream school)?
What is the common practice to follow in USA for such situations? Please help!