I work as a graphic designer, and I printed something for my friend at work. The owner refused to take money for it and said it was a gift, but I took the money from my friend without the owner's knowledge. Is this forbidden (haram)?

All praise is due to Allah.

Whoever is given something as a gift is permitted to dispose of it through sale or by gifting it to someone else, because it has become his property. If your employer exempts you from the cost of something as a gift, you are permitted to dispose of it. If a gift is sincere and not a bribe, it becomes the property of the recipient, who may dispose of it as he wishes. The Sunnah is to reciprocate a gift with a similar one, just as the Prophet, peace and blessings be upon him, used to accept gifts and reward for them. From this, it becomes clear that the saying "a gift cannot be gifted or sold" is false.

And Allah knows best.