A: "Of course, I understand, yet I'm not allowed to make contract changes. I'm sorry I can't help you further. Would you like a lollipop instead?"

or

A: "Of course, I understand, yet we have to follow our [most sacred] process. [...]"

or

A: "Of course, I understand, yet I don't see that button on the screen. [so it doesn't exist in this universe]"

A makes his living playing dumb 8-10 hours a day, it's their job. If B has a life, they'll probably give up at some point.