If there was no written employment or commission contract, the employer could require this: commissions are not protected by wage or labor laws the way base wages or salary are, and without a written contract, the employer may change the terms under which you receive a commission to require this "charitable" giving.But if you have a written contract, it specifies how much (e.g. what % of sales) you get as a commission, then while that contract is in effect (unexpired), the employer cannot alter its terms--you must get that amount without money being taken out for this or another purpose.