Hypothetical

I'm new to the US market but have a hypothetical question. Let's say I come across a promising lead for a property on sale. Being unable for whatever reason at the time to contact the seller directly myself, I pass the lead onto my local realtor to follow up and do the negotiations for me, and gets a Property Agreement under contract to me. I subsequently assign the property to a cash buyer and the property title eventually goes across to the buyer and I collect my assignment fee. How does the realtor get paid? Commission from the seller? Do I pay the realtor fees? Cheers, Russ Swan