[Rebate (ḥaṭṭ) after a Murābaḥah sale]

Answer—and Allah grants success: If the rebate occurs after the buyer has taken possession of the item and the seller has received the price, the murābaḥah seller is not obliged to pass any rebate along. This is what the jurists stated under the chapter of pre-emption (shufʿah) when they discussed what a purchaser must remit to the pre-emptor.
But if the rebate occurs before the foregoing [i.e., prior to the buyer’s taking possession and the seller’s receipt of price], then the seller must grant the buyer a like rebate.
Yes, they also stated in shufʿah that if the “rebate” was couched in the wording of a gift, transfer of ownership, vow, or charity, then the purchaser is not required to reduce the price for the pre-emptor—even if it is a stratagem—whether before or after possession; by analogy, the ruling in murābaḥah is the same.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2