What is the meaning of the Prophet's (peace be upon him) prohibition of selling fruits before their صلاح (ripeness/suitability for consumption) becomes apparent, and the meaning of Ibn Abbas's narration: "The Prophet (peace be upon him) arrived in Medina, and they used to pay in advance for fruits for one or two years," and how can these two hadiths be reconciled?

It is not permissible to sell fruits before their ripeness appears, due to the prohibition of that in the agreed-upon Prophetic Hadith. A salam sale is an exception, and it is a sale of something that is owed, meaning that the seller must provide the agreed-upon quantity of fruit at the time of delivery from any orchard, and it is not a contract for a specific, non-existent fruit. A condition for a salam sale is the full payment of the price at the contracting session. The difference between the two is that the sale of fruit applies to a specific item, whereas a salam sale applies to a non-specific item. The Shari'a has permitted a salam sale despite the slight gharar (uncertainty) present in it.