How do I handle this situation with our realtor?

I've been working with a realtor for about 6 months. During that time we've seen about 20 homes and put in two offers that didn't work out for various reasons. We've been approached by someone that is going to sell their home but is not on the market yet. They want to sell to us directly for sale by owner. We are ok with that. However, we are also looking for advice on how to fairly compensate our realtor for the work they've done so far.