Praise be to Allah.
Once the transaction has been completed between the seller and purchaser, and the meeting in which the transaction takes place has come to an end, then later on the seller claims that he made a mistake in the price he gave to the purchaser, because he usually sells the item for a higher price, in that case the purchaser does not have to give him anything (extra), because the transaction has been completed and all its conditions have been met in full, and it is the seller who must bear the consequences of his mistake.
Moreover it is very unlikely that the seller would make this mistake repeatedly with you for so many times that you cannot remember them, as you said, and not notice it himself until you were the one who noticed it on this occasion.
And Allah knows best.
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