Is it impermissible to sell a car to a third party by Murabaha, then buy it back on the same day and sell it to the original owner, and what should be done in this case?

Whoever stipulates a permissible condition or sets regulations for those dealing with him, it is not permissible to deceive him or circumvent his conditions, based on the saying of the Prophet, peace and blessings be upon him: "Muslims are bound by their conditions."

If the sale of the car to the fund was a genuine sale, and then you purchased it from the fund, there is no harm in selling it to your friend. However, if the sale was not genuine, this is considered a stratagem for usury, as it is as if you took a cash amount from the fund to return it with an increase.