How about following: asking/reference price is determined at the time of signing the contract.

"For every 1% change in the selling price, seller broker will get 10% change in commission rate. i.e.

Sell price Seller broker commission

Reference/asking price 2.5%

1% less than asking 2.25%

5% less than asking 1.25%

10% less than asking 0% (same for anything more than 10% less than asking price.

1% higher than asking 2.75%

5% higer than asking 3.75%