I agree with Trisha. Save some money up somewhere. A secret bank account, whatever. Just don't leave a paper trail around. If you leave the cash in the house, or in the car, chances are it will disappear.
BUT...if I were you, I wouldn't keep this to myself with the plan of dropping a bomb on him. I would let him know you know, and level the field a little bit. Tell him you are not happy with what he's done. Why don't you want him to know you know?
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