Legal Question in Landlord & Tenant Law in Illinois

Technically rent is rent and security is security. You have her money so you have the leg up. You could be "nice" and allow her to use rent to make up the cost differential, but I would recommend that you get her agreement to that in writing. Also, you still have to satisfy Illinois laws about return of security deposit if the kind of housing requires you do so (typically 5 or more apartments in the same building but there are variations on this theme), which could expose you to double return of security just for not handling it properly. So again it's best to get the renter's ok in writing because that kind of agreement can also be a waiver of those other requirement if done properly. Having an attorney write it up would be advisable.