What is the ruling on a Muslim merchant continuously manufacturing wooden coffins for a Christian merchant to export them to European countries?

There is no harm in manufacturing coffins and selling them to Christians, as long as they do not include religiously forbidden items like images of crosses, in accordance with the generality of Allah's statement: "Allah has permitted trade and forbidden usury." Muslims have engaged in permissible dealings with people of other faiths since the time of the Prophet, peace and blessings be upon him.