Actually, if you have a Business Relationship Agreement signed with a Realtor, and on the first Page under Paragraph 2B is filled out with a percentage number or a dollar amount, and you have signed this agreement, then you have already legally agreed to pay your Realtor a Broker's Fee, whether it is paid by the Seller or not.
In other words, if you buy a For Sale By Owner house where the seller is NOT offering to pay any Buyer's Agency fee, you have already legally agreed to pay your Realtor's buyer agency fee out of your owner pocket if it isn't negotiated in the Agreement of Sale to be paid by the FSBO seller. It doesn't matter whether you use the services of the Realtor yourself or not, in the first page under Paragraph 2, it spells it all out fairly clearly that even if you find the house yourself and negotiate everything yourself, if the seller won't pay the Realtor's fee, you agree to pay it to him.
I would talk to your Realtor, tell him you like the FSBO house - he can contact the FSBO seller for you and negotiate on your behalf - often FSBO sellers have COMPLETELY unrealistic listing prices, they don't have a clue how to negotiate, so you're better off with a professional on your side anyway. And also, FBSO sellers often will offer to pay the Realtor Buyer Agency fees if a realtor brings them a buyer, and you'll still get the house you like and get the negotiating services of a professional who can make sure you also don't get hosed at the settlement table, all without it costing you a penny. And if the seller won't pay the agency fee, you still have the option to pay the Broker fee yourself if you still want the FSBO house.