What is the ruling on an employee combining a fixed salary with a percentage of profits (bonus)? Does this invalidate the contract and make the salary unlawful, or does it only make the bonus unlawful?

It is permissible to operate under an annual bonus system for employees when it is mandated by force of law, and it is permissible according to a narration in the school of thought of Imam Ahmad. It is also permissible if it is an agreement made willingly by both parties. This employee is entitled to the wage of his like for his work, even if his rental contract is ruled invalid.