What is the ruling on reducing the weight of a bag by only 10 grams from its actual weight of 15 grams when weighing a customer's order at a coffee shop, given that the customer is unaware of this?

If the seller knows the weight of the larger envelope, he should inform the buyer that the envelope is 10 grams less, not 15. If the buyer agrees to this, the sale is valid. Scholars have differed on the permissibility of selling solid substances along with their containers, provided the weight of the container is deducted. Imam Al-Nawawi deemed it permissible and attributed this view to the majority of scholars, arguing that it involves no corruption, deception, or uncertainty. Imam Ahmad also permitted such a sale. For this sale to be valid, both parties must know the weight of the container; otherwise, the sale is void because ignorance of its weight renders the item sold unknown in quantity.