Is it Islamically permissible to back out of a real estate sale that was agreed upon over the phone and for which a down payment was made, then it later became clear that the selling price was lower than the actual market value at the time of agreement, due to the seller, who resides abroad, being unaware of real estate prices?

It is permissible to retract a promise to sell, as it is not binding according to the majority of scholars.

As for a firm sale, if the offer and acceptance occurred verbally, it becomes binding after the parties separate, and it is not permissible to retract it unless there is a reason for an option (khiyar), such as a condition, a defect, significant deception (ghabn), or mutual annulment (faskh).

If one of the contracting parties finds that they have been significantly deceived (ghubina) in the price, they have the option to either annul the sale or uphold it. Gross deception (al-ghabn al-fahish) is determined by the custom of merchants, and some jurists considered deception to be one-third or more of the price.

The majority of jurists restrict the option of deception (khiyar al-ghabn) to specific situations where there is clear misrepresentation. The Malikis, in one opinion, held that the option of deception is established for anyone who has been deceived if they were ignorant of the true value. Some of them restricted it to one-third, and this is the preferred view.

Therefore, if there is a significant difference between the price and the market price, and you were ignorant of the market price, you have the option to either annul the sale or uphold it. Otherwise, the sale is binding, and it is recommended to accept rescission (iqalah).