What is the ruling on selling orange fruit (like mandarins) on the trees before they turn yellow, knowing that there isn't much time left until they turn yellow (around 20 or 30 days) and reach full ripeness?

It is not permissible to sell fruit before its ripeness becomes apparent, whether it is oranges or other types of fruit. Such a sale is considered void due to the presence of gharar (excessive uncertainty), and the Prophet, peace and blessings be upon him, forbade it. In the two authentic collections (Bukhari and Muslim), it is narrated from Ibn Umar, may Allah be pleased with them both: "Indeed, the Messenger of Allah, peace and blessings be upon him, forbade the sale of fruits before their ripeness became apparent, forbidding both the seller and the buyer."