You need to consult an attorney for legal advice. This is not legal advice.
You should be able to write a contract that excludes the seller from paying a commission IF this specified person buys the property.
However, you might want to have some representation in this transaction (unless you like gambling).
Also, normally in the listing agreement it specifies the protection period (the duration) and when the Realtor is required to submit a list of those people who are "protected".
Aside from the legal aspects, the moral perspective is that if this broker brought you a buyer, with whom you may have said something like "So, if we omit the Realtor from this transaction, we can do this cheaper for both of us", I personally have some issues:
1. So you want to avoid paying someone a lawfully earned commission? How would you feel if someone did that to you?
2. You are considering having the broker sign s NEW listing agreement and EXCLUDE this person? Every though that broker brought the buyer?
In Rotary Interational we have a Four Way Test
Of the things we think, say, or do
First, is it the truth?
Second, is it fair to all concerned?
Three, will it build goodwill and better friendships?
Fourth, will it be beneficial to all concerned?
You might want try applying these questions to your situation.