Home Selling Details
If you know you have a potential buyer before you sign a listing agreement, you can ask the agency to exclude him from the agreement. If an excluded buyer makes an acceptable offer, you don't owe a commission, no matter what type of listing you signed.
The agency will probably not agree to exclude the buyer for the full term of the listing, but might agree to a short exclusion to allow him time to make a decision. It doesn't hurt to ask!
Read more details about Different Types of Listing Agreements