What is the ruling on a graphic designer taking the price difference between what he charges the client and what the printing press charges, knowing that he brings the work to the printing press?

If you are an agent for business owners in dealing with a printing press, you must seek the lowest price for them. It is not permissible for you to take an increase over what the printing press determines unless you have their permission, and you must return any previous increases to them. However, if you agree with them on a specific price, then agree with the printing press on a lower price and take the difference with the consent and permission of your employer, there is no harm in that. It is permissible to agree with the principal on a fee for the agency.