Is it obligatory to pay the deceased's debt of 4000 Riyals to the merchant, knowing that the deceased believed the merchant defrauded him by selling him the goods at several times their actual price?

There is no limit to profit in trade. If someone buys a commodity knowing there is an excessive overcharge, they have no right to annul the sale because there was no deception. If the sale was based on bargaining and negotiation, the buyer has no excuse for paying the price, even if it was many times the value of the commodity. Some scholars have stated that if the buyer was unaware of the value, sought the seller's advice, and the seller sold it to him at an exorbitant increase (more than a third), then the buyer has the right to annul the sale. However, if the buyer is a merchant and has experience with prices, he does not have this option. Assuming he did have the option, the obligation would have been to annul the sale and return the commodity, not to refuse to pay the remaining price under the pretext of overvaluation. The debt must be paid from his estate unless the merchant waives it.