Is the seller entitled to deduct the value of the penalty clause (50,0) upon فسخ the contract and refunding the amount, after the buyer discovered that the agreed-upon price of the land (504,0) is much higher than the real market price (336,0)?

A penalty clause between buyer and seller is permissible, unless there is a legitimate Shariah reason for annulment, such as the option of excessive deception (Ghabn Fahish). Excessive deception is determined by the custom of traders, or if it amounts to one-third [of the value], or more than one-third.

If deception is proven, the buyer is given the choice between returning the land and taking back the price, or keeping it without compensation (the difference between the price and the market value).