Unlike loans, grants do not usually have to be repaid. However, there are two reasons why you may have to repay part of a Federal grant:
1) The amount given to you was more than you were eligible to receive (this is called an over-award). This can happen if the school makes an error when calculating your eligibility for financial aid, or if an audit of your financial records reveals that some of the information you provided was incorrect (for example, your income was higher than what you reported on your application for financial aid).
2) You withdrew early from the program for which the grant was given to you.
Source: US Education Department