What is the ruling on invoking God's mercy upon a specific person after a certain event, such as saying: "May God have mercy on So-and-so, for this product was cheaper in his time," and does the ruling differ if the person is mentioned negatively?

If a person was a Muslim, it is permissible to ask Allah to have mercy on him, and it is forbidden to speak ill of him because that constitutes backbiting, which is prohibited. However, if a person was a disbeliever (kafir), it is not permissible to ask Allah to have mercy on him. There is no harm in speaking ill of him if he was a disbeliever who waged war against Allah and His Messenger.