Article or money handed over first

If the article is sold in exchange for currency, then it will be told to the buyer to hand over the money first, when he has done this then it will be said to the seller to hand over the article of sale.
If one commodity is sold in exchange for another one or one currency is exchanged for another, then it will be said to both the contracting parties to hand over their respective items simultaneously (Kitaabul Buyoo).