Is it permissible to donate fifty riyals instead of giving them to a qat seller, knowing that qat is haram and prohibited in Saudi Arabia?

The ruling regarding settling the remaining price of khat that is owed by you varies based on your beliefs at the time of the contract:

1. If both of you believed its sale to be permissible at the time of the contract: You must pay the remaining price to the seller, because the sale was valid according to your belief at that time. 2. If both of you believed its sale to be impermissible at the time of the contract: You are not obligated to pay the price to the seller. Rather, you must donate the remaining amount as charity, so that you do not combine benefiting from the khat with its unlawful price. 3. If you believed it to be impermissible, and the seller believed it to be permissible at the time of the contract: You are obligated to pay the price to the seller, because the seller considers it permissible wealth for himself.

It is recommended to inform the seller about the impermissibility and harm of khat, and to advise him to stop selling it and to donate what he earned from it as charity.