Is it permissible for the buyer to withdraw from her agreement and demand the full amount, after she had agreed to take a portion of it as compensation, knowing that the seller has not yet received her compensation from the fraudulent merchant?

If the questioner is an agent acting on behalf of the employer, then if the buyer knows that she is an agent, the questioner is not liable. If the buyer does not know, the questioner is liable for the price to the buyer. However, if she is a mediator, she is not liable unless she cheated or deceived. In the event that the questioner is liable, if a settlement is reached to waive part of the price, it is not permissible to revoke it.