Is it permissible to sell an item at its original price instead of the discounted price to a customer who procrastinates in paying debts?

There is no blame upon you for selling the commodity at the current market price, not at the previous preferential price, whether for the reason mentioned or for any other reason. The contract is permissible based on the price agreed upon by the buyer and seller, for Allah Almighty says: "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent," and the Prophet, peace and blessings be upon him, said: "Indeed, selling is only by mutual consent."