The seller is looking for their best net price. It sounds like you are ether dealing with an XA or a FSBO that may not know the MLS rules about published commissions. Spokane has a different MLS than we do in the Seattle market, so the rules may be different, but I'd be surprised. If the home is in Olympia, it would be governed by NWMLS rules, which prohibit this, unless itâ€™s a FSBO.
My suggestion is to prepare a net sheet with whatever commission is advertised and explain that the commission is between you and your buyer. Some companies have been rebating commissions to buyers, so he may be aware of that.
Unless you are dealing with a multiple offer scenario and want to reduce your portion to give your buyer a better position, which I don't advise there are better ways to do this, my answer would be no.
If you don't get to a binding contract and have written evidence of this, I'd consider going after your commission, assuming you're the buyer's agent.