If your client arbitrarily and unilaterally changes your billing terms when signing the proposal (I hear about this regularly) then you don’t have a trusting, peer-level relationship. Partners don’t do that to each other. But buyers frequently try to take advantage of “venders.”
Just because someone doesn’t kick you out of their office and agrees to read a proposal doesn’t mean you have a solid relationship. Make sure you do before plunging onward.