Change Orders With No Markup?

To be fair the tile isn't completely accurate (maybe a little click bait-ish). However it's close...

We have a project that will not allow us to charge our standard hourly rate on a change order. I should mention that they have already agreed to the additional cost that we quoted them for the change order. However now we have to fill out a form that they have which breaks down the cost. And now they are saying we can't charge what we were charging.