Is working in a store that provides installment payment methods through financing companies or credit cards at prices higher than the cash price impermissible, bearing in mind that this might be considered condoning falsehood and assisting in wrongdoing?

The prohibition of void sales falls upon the one who transacts them or anyone who directly and intentionally assists in them. If these sales occur in a shop where the inquirer works, the responsibility falls on the customer and the financing entity, as well as those who assist them. There is no blame on the seller (the shop owner or manager) if he has no connection to the void contract. However, if the seller is a third party without whose involvement the transaction cannot be completed, then it is not permissible for him to do so. An employee in the shop is permitted to work unless he directly participates in the forbidden contract or directly assists in it. Advising people not to purchase through forbidden means is an obligation upon everyone who knows what is forbidden, to the best of their ability.