Three ways:
(1) Let the agent sign an agreement that he will give you a few thousand dollar cash back (2% ?) if you use him as an agent to buy this house.
(2) Make a very very low offer (10-20% cut). Because the agent knows he will get all of the 5% commission, he will do the best to push the seller. If the seller is manipulated by the agent, the deal usually can be done.