I respectfully disagree. If your intention was to do it as merely a favor, there is no financial obligation.
Chein seems to disagree with you. He said:
you have to pay what halacha dictates not what you feel is the proper amount. Same for all halahic brokers/service provides.
My understanding of his statement is that even if I feel that zero is the proper amount, I’m still required to charge the amount halacha dictates.
The Wolf