Legal Question in Business Law in California

Oral agreements are difficult to prove, but they are enforceable.

If you have fulfilled every condition for the retention bonus, your employer has an obligation to pay it to you. Failure to pay would be a breach of contract.

Sure, but how do you prove the details and terms of the agreement if you don't have it in writing, and he denies the terms?

If serious about hiring counsel to help in this, and if this is in SoCal courts, feel free to contact me. He may be willing to compromise if you hire an attorney with the implied threat of litigation.

Oral agreements are usually enforceable as a contract, if their terms satisfy the requirements that apply to written contracts. As others have noted, the obstacle is usually proving what the contract said. If you had multiple witnesses, then this may not be much of a problem in your case.

Some types of contracts must be in writing, but yours does not appear to be one of those types. Different limitations periods often apply to oral contracts and there may be some other procedural differences. Offhand, though, I don't think these issues would matter much in your case.

You should consult with an attorney ASAP in order to better assess your rights.