Legal Question in Civil Litigation in California

Yes, you have recourse available to you. You will likely claim an oral contract and its subsequsnt breach.

These can be tough cases - a he said, he said conflict. Your basic argument is that the check issued to your cousin was based on business motives only and intended as a gift.

You may need to contact an attorney. Some may be willing to take the case on a contingency fee basis.

I suggest you speak to an attorney at length, at least to get some consultation. It is not clear from your post what the full terms of the contract were, and you may or may not have a statute of frauds problem. Additionally, you may also have some issues with jurisdiction and choice of law, such as whether Colorado or California's law applies.

Sure. You now get to try to enforce an oral contract, evidenced only by the cashed check and 'your word' that this was a loan and not a gift. You'll probably have to sue him in his state, making it inconvenient for you to go to court for hearing[s]. If that state has a small claims court money limit high enough to cover your check, you won't need an attorney. If you have to file in their Superior Court, you will need an attorney. Good luck. Next time, properly document your transactions. Phone calls are of absolutely no value, as your version of the conversations are not admissible in court. Now, write him a detailed letter laying out your version of the transaction in detail, and demanding whatever you want him to do, on the threat of lawsuit.