So you were effectively requesting that they advance you up to thirty days of sick leave, which you had not yet accrued? And they declined your request?

The ADA provides certain guarantees of unpaid leave time - so at first blush it does not appear to be an ADA issue for them to deny you an advance on future sick leave. You may be able to show discrimination based upon the nature of your disability, but you haven't provided any facts which would indicate that to be the case. You would be well-served to have your situation reviewed by a local lawyer who handles plaintiff-side disability and employment claims.