IAAL (this is not legal advice). An offered but undelivered gift conveys no legal obligation on the would-be donor. In order for it to become an enforceable contract, some sort of additional consideration would need to be required from the employee over and above their existing work (e.g. overtime, extending a contract, etc.), and a promise to do that would need to be accepted from the company.

This is covered in first-year contract law.

That said, an employee might be able to recover from injuries on the basis of promissory estoppel if they relied on the promise of a bonus to their detriment.