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Are you a licensed, full time agent in the area? Depending on the rules in your state procuring cause may be a factor.

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Of course the listing agent isn't properly looking out for your friend's interests. He sounds like an experienced enough investor that he should have known that up front.

I also tend to believe the listing agent is bluffing, though the listing agent's commission is between the herself and the seller (and therefore really none of your friend's business). With that said, it does sound like she holds rightful claim to the procuring cause either way. I certainly wouldn't be too inclined to pay a commission to an agent who suddenly appeared mid-negotiation.

There's certainly nothing to prevent you from offering your services to your friend for a fee (i.e., that he--and not the seller--would pay for). That's probably your best bet to avoid a potentially nasty and lengthy procuring cause dispute.