Paying back a scholarship award

I was awarded a scholarship of $400 per quarter to attend a state run university here in Ohio. The $400 was put into my school account that was in turn routed to my personal checking account. Summer and fall quarter of 2000 I was forced to withdraw from classes because of a very hectic schedule at my full time job (also at the university). So of course I must now pay back the scholarship money that was dispursed to me for those 2 quarters the total being $791.00 ($9.00 was deducted to pay for the mandatory bus fee). I never received written notice that I needed to pay this money back. I was on my university's web site when I found out that they had placed a hold on my account. I wrote a letter to the accounts receivable department that I would send a check in the amount of $50.00 every 2 weeks until the debt had been paid off as this is all that I can presently afford. My question is that they are trying to charge me an 18% "finance charge" on the total amount is this legal? This was never disclosed to me when the scholarship was awarded, and I never signed anything to receive the scholarship, it was automatically dispursed to my account. I should also mention that this is not a scholarship that I applied for, I received it soley on the basis of my ACT scores. I would be greatful if anyone could please let me know if it is legal for them to do this. Thank you in advance.