Larry you need to be more specific. Is the listing agent saying, if you use him as your agent as well he will give you 20% of his commission? Or is this an agent that will represent you the buyer on any home and he will give you 20% of his commission? Please specify.

In either case I would recommend you proceed in the following manner to reduce any exposure to taxes. Keep the rebate in escrow. That is, simply ask whichever agent who is offering this, to go ahead and reduce thier commission by the 20% they offered. They should submit a CBC (cooperating broker commission) form to the listing agent in order to do this. This will enable the seller to reduce the payout of commission by 20%. In parallel, you should have an addendum that is submitted to the seller to credit you the buyer that same amount at close of escrow for non recurring and recurring closing costs. In this manner you will not receive a 1099 from the broker.