Sale by Mulamasah (Touching) and Munabadhah (Throwing)
It is authentically established from the Prophet ﷺ that he forbade the sale by mulamasah, the sale by munabadhah, and the sale effected by throwing a pebble.
This is because it involves deception.
Meaning of mulamasah: The seller says to the buyer: Whatever cloth you or so-and-so touches, you must buy it for this much money.
Meaning of munabadhah: The seller says to the buyer: Whatever cloth I or so-and-so throws towards you, you will definitely have to buy it for this much money.
Sale by pebble: The seller says: Whatever piece or whatever cloth the pebble falls upon, you will have to buy it for this much money.
Any transaction that resembles such a dealing will fall under this ruling because of the presence of this gharar (deception), since the buyer did not enter into that transaction with complete knowledge and insight into the true nature of the goods.
Allah, glorified is His majesty, is more merciful to His servants than they are to themselves, so He has forbidden them from every act that would cause them harm in their dealings.