What is the ruling on the seller estimating the value of what the customer ate from the goods before purchasing them and deducting it from the weight without informing him, in order to guarantee his trustworthiness with the employer?

It is permissible to taste goods whose quality and taste vary, and this affects their price and desirability, in a small amount that fulfills the buyer's need, due to common practice. Some scholars have explicitly permitted it, and some have stipulated obtaining permission. If the buyer exceeds the customary amount, the seller must prevent him or inform him that the tasted amount will be deducted from the price.