What is the ruling on a paint shop owner agreeing with workers to give them a percentage of his personal profit in exchange for them purchasing paints from him, without increasing the price for customers? And does this fall under the ruling of workers' gifts? And what is the criterion for what is permissible and impermissible among them?

It is permissible for a broker to take a known and predetermined commission for mediating a sale, unless there is deception or fraud against the buyer. If the paints are of suitable quality and the worker described them truthfully, then there is no blame upon him for taking a commission from the seller. However, if the worker is paid a wage for purchasing the paints, it is not permissible for him to take a commission from the seller without the employer's permission, because this is considered among the gifts given to workers.