Is it permissible to sell merchandise at a price lower than that of the authorized agent without his knowledge, and what is the ruling on mediating its sale?

There is nothing that obliges the questioner to adopt a specific fatwa (definition) and act upon it. If the questioner is not convinced by the fatwa given to him, it is permissible for him to seek another fatwa from a different scholar. If he acts upon the fatwa of a reputable scholar, there is no blame on him, especially if he believes that this opinion is more sound and stronger in evidence. Imam An-Nawawi (may Allah have mercy on him) said in Al-Majmu': "A layman is not bound to adhere to a specific madhhab (definition). He should seek a fatwa from whomever he happens upon. If he is given a fatwa, he is bound to act upon it. He is not allowed to seek a fatwa from another just to find what suits his desire. If he seeks a second fatwa, and it contradicts the first, he is not bound to act upon either of them. He should then refer to a third mufti (definition), or he may choose and act upon whichever he wishes. The correct and chosen opinion, which the imams, most jurists, and researchers hold, is that he is not bound to adhere to a madhhab; rather, he may seek a fatwa from whomever he wishes. If he is given a fatwa, he is bound to act upon it in that particular issue. It is also said: It is permissible to seek a second and third fatwa and act upon what is stronger in evidence in his view. This latter opinion is the correct one." [End of quote]

Ibn Abidin said in his commentary: "If a mufti gives him a fatwa and he acts upon it, then another mufti gives him a different fatwa from the first, he may act upon the second fatwa as long as his intention is not to seek out dispensations." [End of quote]

And Allah knows best.