Is contributing to a charity for cancer patients—which deducts 15% from contributions to provide a gift (a car or a house) to one of the contributors through a lottery as a form of incentive—considered gambling?

It is not permissible to contribute to this association because it is based on gambling. The car or house is not a symbolic gift, but rather a considerable sum of money that makes the transaction an exchange, not a pure donation. The contributor pays with his eye on the car or house, and this is gambling. Deducting 15% from donations for the sake of the gift is a bad act. If the donors did not authorize it, then it is an illegitimate act. If they did authorize it, then they are gamblers. The obligation is to cancel the gifts and encourage donation for the sake of Allah Almighty. If the association continues with this system, then it is not permissible to donate to it.