The fact that a person bought the raffle in the first place was their way of giving tzedakah to that mosad. When a person buys a raffle ticket (usually more than one) they don’t really expect to win, so their intention is usually to give tzedaka to that mosad. So no the mosad does not have a right to automatically assume anything. Unless it is written upfront in the rules of the raffle program, they have no right to deduct anything and by law have to remit the entire amount or be liable for fraud.
A person has the right to donate masser to whomever they choose. As a matter of fact they might have met aniyim at the auction that they decided to donate to if they happened to win, or they might have had intention of participating in the auction/raffle to begin with only to donate the money or the masser of the money to a certain individual or organization. So by taking the money out it is not only chutzpah it probably borders on gezeilah if not genaivah. The intention here is for the mosad to give and not to take!