No its not binding on you to pay the brokerage. If he has introduce you as a seller.
If he wants brokerage then he should have introduce as a agent to you why as a partner?
You can show him his advertisement published in newspaper and ask him to read the terms and conditions.
If your deal is finalized then don 't pay brokerage to him. He should have informed you before the deal so you could have negotiated price of the plot.