It is your word against theirs about what the final discussion was.
You will need to confirm in writing what your discussions with her husband were and that you completed the sale based on the verbal agreement that you could charge the $500.00 which you did.
The agreement you have is clear that you will charge a fee for selling it and you have amended the agreement verbally. The court if it goes down that path will not say you have to give them the $500.00 as you have a right to charge a fee.
But you should make sure that you now confirm the verbal agreement with her and advise that should she take it further you will defend the claim and also seek costs.
This is your only option.
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