What is the Islamic legal position regarding buyers after selling olive oil that was later found to be adulterated, and what should be done about the ongoing installments?

If it becomes apparent that the oil you bought is adulterated, there is no sin upon you if you were unaware of it. However, you are obligated to inform those to whom you sold it. They then have the option to either rescind the sale and return the oil, if it is still available, or keep the oil and be compensated for the defect (the difference in price between the adulterated and the sound product). If the oil is no longer in their possession, they are only entitled to compensation for the defect, as return is impossible. This ruling also applies between you and the original seller.

Ibn Qudamah mentioned in "Al-Mughni" that whoever discovers a defect in the sold item of which they were unaware has the option to either keep it or rescind the sale, whether the seller knew of the defect and concealed it or not.

He also added in "Al-Mughni" that if one buys a defective item and then sells it, their right to return it is forfeited. If it returns to them and they wish to return it due to the original defect, then if they sold it knowing of the defect, or if something was found that indicates their satisfaction with it, they do not have the right to return it. However, if they were unaware of the defect, they have the right to return it to their seller.