As long as you follow the ABA and Illinois State Bar rules mentioned above, your fee arrangement with another lawyer is enforceable. On the other hand, violating the requirements renders a referral fee agreement void and unenforceable.
Violations that could lead to an unenforceable fee agreement include:
Failure to put the agreement in writing.
Failure to inform the client about the referral fee and arrangement.
Failure to obtain the clients consent to the fee arrangement in writing.
To ensure enforceability, get your client to sign a clear disclosure agreement.