«الْبَيْعُ أَصْلٌ فِي الْبَيْعِ، وَالثَّمَنُ شَرْطٌ فِيهِ»
The article sold is the primary object of the sale; the price is attached as its consideration.
Ba connects the price or substitute to the object sold. The two sides of a sale do not therefore have an identical legal position for every purpose. The destruction of the price and the destruction of the sold article can have different effects.
The interpretive task is to distinguish selling a particular thing from specifying another thing as its price. The particle expresses the link between those two roles.