Can you explain the circumstances some more?
Thank you for the additional information. Based on the information you have provided, I cannot see how he can charge go a fee. You agreeing to 'look after' him is not a binding agreement and in any event, in the absence of a written agreement, it can be argued that there is an implied agreement however, these are notoriously difficult to prove and if the matter was to go to court, the courts will look at any correspondence surrounding the circumstances.
I would suggest that you do not pay him anything whatsoever. The general practice in the UK is that an agent would charge the seller a fee for marketing the property and finding a buyer, fees are not generally charged to purchasers.
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He may be acting as an agent for you on the sale of your property, but I cannot see how there can be a binding agreement in relation to your purchase. Therefore, my view still stands.
In order to have a contract the three elements of offer, acceptance and consideration need to be present. In your case, I do not believe there was an acceptance on your part.
Therefore, my opinion still stands. If he wishes to recover anything from you, he would have to issue proceedings. It would then be for a judge to decide.