First, congratulations on a loyal buyer,
Jamar. Not that common in this business. You're doing something right!
I'm not a lawer but the listing agent can claim
"procuring cause" - even without a signed representation. Check with your broker and/or attorney on your local rules and laws.
I paid a referal fee in a in similar situation. And gladly

Everybody wins: the listing agent sells the listing, you sell a house, your buyer gets a new home, you both get paid, and you'll maintain a good repuation. That alone is worth a few dollars.