No you cannot keep your ex-employer's tools. Even though you are owed money, you cannot take their property as collateral for payment. If you do not return them, he can file a criminal complaint for theft or "conversion" (this is when a person lawfully obtains possession of another's property but then converts it for their own use and without the owner's permission). What you need to do in order to receive all compansation owed you is to sue your former employer in small claims court and/or file a complaint with your state's department of labor.