What is the religious ruling regarding a client deducting more than 45% of the price of goods previously agreed upon, due to the presence of another supplier with a lower price and the appearance of an unseen defect, knowing that the seller agreed to the deduction out of embarrassment and has not yet received the remaining money?

The transaction is valid as long as you agreed to a price reduction due to the defect. You can demand the remaining amount from the seller. It must be noted that the fundamental principle is the impermissibility for a seller to conceal defects they know of from the buyer, because this constitutes deception and fraud, and due to the Prophet's (peace and blessings be upon him) saying: "Whoever cheats is not from me."