Is it permissible for a scholarship student to receive two salaries from two different entities (the scholarship-sponsoring entity and the employer) for two different jobs, knowing that his second job will not affect his academic scholarship?

You are obligated to fulfill the contract with the supervisor and inform him about the return of the withheld salary, for Allah Almighty says: "O you who have believed, fulfill [your] contracts." And as it came in the Hadith: "Muslims are bound by their conditions, except a condition that makes a lawful thing unlawful or an unlawful thing lawful." So it is not permissible to conceal the matter from him. If he agrees to the situation remaining as is, then that is good and excellent. Otherwise, do not conceal it, because that is a forbidden deception, for it has been narrated: "Deceit and treachery are in the Fire." And deception contradicts sincerity and purity of hearts and cultivates sin and transgression.