وَمَنْ بَاعَ سِلْعَةً بِثَمَنٍ قِيلَ لِلْمُشْتَرِي: ادْفَعِ الثَّمَنَ أَوَّلًا، فَإِذَا دَفَعَ قِيلَ لِلْبَائِعِ: سَلِّمِ الْمَبِيعَ
Translation: When someone sells an item for a price, the buyer is instructed to pay first; after payment the seller is instructed to deliver the sold item.
Silah means the sold commodity and thaman its monetary price.
For an immediately payable cash transaction, the classical ordering is payment first, then delivery of the identified goods. The commentators explain the sequence by the legal status of the monetary price and the already designated sale object.
A deferred-price contract or unavailable item creates a different delivery question.