Is it permissible to reclaim a sum of money, exceeding the original price of the land, that the broker had unlawfully taken, by deducting it from the remaining amount due to him after his death?

Ghabn (unjust enrichment/fraudulent overpricing) is an increase in price beyond the market value. It is divided into excessive (fāḥish) and minor (yasīr). Excessive ghabn establishes the option (khiyār) in some cases, while minor ghabn has no consequences. The limit of excessive ghabn is determined by the custom of merchants, or what cannot be appraised by appraisers according to the Hanafis, or what exceeds one-third or one-fifth according to others. 12% is not considered excessive ghabn. The option of ghabn is established according to the majority of scholars if it is accompanied by deception (taghrīr) from the seller. The Malikis and Hanbalis held that the option is established for the "mustarsil" (one who does not know the value of things and is not good at bargaining), even if there is no deception. The option of ghabn is exercised by returning the commodity and taking back the full price, not by taking the difference (arsh).

In summary: You do not have an option in this case, and the deducted amount must be reimbursed.