Is what a graphic designer does—offering a printing price to his company at market rate, or less, then printing at a lower price than the agreed-upon price, or receiving a commission from the printing press without increasing the price for his company—permissible or forbidden? And does the same ruling apply to dealing with other institutions?

It is permissible to sell designs to a company other than the one you work for under two conditions: the company's permission and that the work is done outside official working hours. Selling is prohibited if the company forbids it or if the work is done during working hours. As for mediating between the company and printing presses and taking a commission, it is not permissible except with the company's permission.