Is it permissible to renegotiate with someone who has committed fraud regarding the price after an agreement, and is there any legal (Sharia) basis for that?

If the agreement to sell is still in the preliminary stage, it is not binding. However, if the contract of sale has been concluded, it is binding on both parties and cannot be revoked except with the consent of the other party.

The price being significantly higher than the market price does not, by itself, constitute fraud. Scholars have differed regarding the establishment of the option of ghabn (excessive overpricing/underpricing) for the buyer if he was ignorant and was subjected to an uncustomary ghabn.

The seller can be encouraged to revoke the sale by reminding him of the desirability of iqalah (cancellation of a sale by mutual agreement), based on the Prophet's (peace and blessings be upon him) saying: "Whoever cancels a sale for a Muslim, Allah will cancel his stumble (forgive his sins)."