My former boss and I had an oral agreement for me to purchase a truck belonging to the corporation I worked for.

Oral contracts are, in most cases, valid in California. No, your former employer may not keep the payments unless you had an oral agreement otherwise. The problem is proving there was an oral contract for the purchase of the truck and proving that you paid him $800 towards it. If you have cancelled checks or deductions from paychecks that can show you made those payments, it may help your case. You can always file a lawsuit in small claims court for the $800, but the burden of proving the existence of the contract and the $800 in payments is on you. Good luck.

Based solely on what you've stated, no, he cannot do this. Mr. Keystone has it right. That said, he fired you for something. Perhaps there is more to the story than you are letting out? If not, your avenue for recourse is small claims court.

No, that's theft. The proper place to turn is small claims court.