All commissioners are negotiable and there's no mandatory structure. Sometimes -- but rarely in this market -- the Buyer's Agent does get a flat fee (like $20k flat on a $500k property if it's a dog). It's just tradition and simplicity that make a percentage split the typical case.

You can always ask a Buyer's Agent to work on an effective flat-fee basis for a pre-arranged amount. If the commission they actually get is lower, then you make up the difference out-of-pocket. If it's higher then they "refund" the difference in a lawful way that also comports with loan requirements. But "lawful" and "loan" aspects can make this approach cumbersome.