Tuition Reimbursement Contract- Illinois

What is the name of your state? IL

I am currently participating in a tuition reimbursment contract with my employer where I've agreed to stay with my employer for a duration of 3 years after payment recieved from them.

I am recieving full tuition reimbursment (over the 5,250 tax-free limit) and recieve payments approximately 2-3 weeks after course completion and verification of grades.

My question - Is this contact legal in Illinois? A labor law proffesor once mentioned that some jurisdictions have ruled that such an agreement creates a situation tantamount to involuntary servitude, and thus these contacts are not enforceable. In these jurisdictions, he goes on to say, the only way to get around this issue is to implement verbage in the contract describing the tuition reimbursement program as a "loan" of zero percent interest, which can be forgiven when the employee has worked X amount of years. My contract with my employer has no such verbage

Further background info: I did sign a contract saying that I would either pay back the full amount of the tuition reimbursement or a pro-rated amount (if I left between 1-3 years) if I did not stay for the full 3 years. Finally, this agreement I have with my employer is not company policy- our company policy is a limit of $5250/yr for a 1-yr retention. - My situation has been deemed a "special case"

So then-
1. Is the contract I have with my employer enforceable?
2. If yes, how does this work out with the monies in excess of the 5,250 tax free limit?
- since I've paid income/state/s.s. taxes on these amounts (which are substancially large), how would I go about getting back the taxes I've already paid?
- I would imagine that any income which has been taxed can not be rescinded.

Your opinions?