Is it a sin to respond to a customer's request by paying him a commission from the agreed-upon price increase, knowing that he threatens to terminate the deal if refused?

The questioner must refrain from giving a commission or a percentage of the price to the agent authorized by the owner if the agent takes it for himself without the knowledge of his principal, because that falls under the category of aiding in the unlawful consumption of wealth, even if it leads to ceasing dealings with him.