Is it permissible to work for intermediary websites that take a percentage of the profit, varying according to the income, in exchange for providing job opportunities?

The basic principle is that the website's commission should be specified and known. However, it is permissible for the commission to be a percentage of the earned income, even if it was not known at the time of the contract. This is because such an ambiguity does not prevent delivery, and it is excused because it will eventually become known.