Commission for an agency

Hi, I am an illustrator. I have a commission to create pictures for a client through a communication agency. I gave my price and my conditions to the agency at the beginning of the work. This mentions that I need a briefing. I realize a sketch according to the briefing. Then I correct the sketch if the client wants modifications. If the client wants modifications that are not included in the briefing, I charge an hourly rate for this work.

This is what is happening. The client wants some changes that were not included in the briefing. But I’m charging to the communication agency. The two people from the communication agency say they can pay me from what they are earning, but if I charge more, they would lose their own personal money. They said: “I would payout from my own pocket.” I felt really embarrassed. They don’t want to ask for more money from the client.

I think that I want to honor my boundaries, and the way I want to work. But if I charge more, I would feel guilty, and I’m afraid I will lose the opportunity to work with this communication agency again.

C client from communication agency says: “I would payout from my own pocket.”
T this is impossible to ask her to pay me
E embarrassed
A I say: “I understand, I’ll see how much time it takes at the end”, I judge the communication agency and the clients
R I take from my time and my money to finish this work, I don’t honor my conditions

And it seems that I totally underestimated the time this job would take.
I feel kind of angry against them, kind of stuck, and surely not motivated anymore to draw those illustrations.
Would you have an insight? Thank you!!