One of the reasons brokerages do not enter into contracts with FSBO (buyers/sellers) is because there is a serious liability incurred by the brokerage when the other side of the transaction is not represented. I say this because you mentioned that the other party does actually have an agent. (I will return to that point in a minute)
As for what commission you want to offer that agent as a coop commission that is entirely up to you. There is no legal/ethical requirement. The “average” coop commission is 2.5 or 3%. I am not saying this is legal/ethical, or reasonable, I am just stating what the common payment is.
Remember all commissions are negotiable. You can offer what you want and the agent can take it or leave it.
Getting back to the first thing I mentioned, if the broker that the agent works for doesnt want to risk dealing with a FSBO you will not have any decision to make at all. You may want to try to contact the buyers and deal directly with them but they may feel much more comfortable having professional representation.
In that case you may want to try to strike a deal with the agent letting the agent be a dual agent and represent you as a listing agent (for these buyers only) at a low commission. Then pay him the listing and the buyers commission as well.
