What is the ruling on selling on behalf of a boy in his father's shop?

Question:
He says: Honorable Shaykh, may Allah grant you success, fathers often place their sons, who are young and not yet adults, to sell on their behalf. Is this sale valid?

Answer:
It is not valid. The seller must be of sound mind and capable of managing his affairs, and a minor is not capable of managing his affairs. If the father wants to test them, to see if they are shrewd or not, there is no harm. Test the orphans. The test is that as the child approaches puberty, he is returned to buying and selling to see if he is shrewd or not.
If he is not shrewd, and if the intention is that the price is fixed and written on the item, then when the customer comes, he pays the fixed amount to the child and takes the item. This is permissible because there is no gharar (excessive uncertainty in a contract). However, if the child haggles, bargains, lowers the price, or raises it, this is not permissible. Yes.