What is the ruling on contributing to the construction of a children's amusement park that contains impermissible elements (such as gambling, music, unlawful singing, immodest display, and unveiling), knowing that my work is limited to supervising the construction of the permissible games within it (such as trains and water attractions)?

Conflict has arisen even regarding what the questioner mentioned as undisputed. Disagreement has occurred concerning selling grapes to someone who will turn them into wine, and renting out a service to carry wine for someone who will drink it. According to Abu Hanifa, the payment for carrying is permissible because the act of carrying itself is not a sin. Similarly, disagreement has arisen regarding renting out a house to be used as a place for illicit activities (like a “house of fire”), or a church, or where wine is sold, as well as selling a beardless boy to someone who will commit indecency with him.

However, the majority opinion is more sound in its foundational principles, derivations, and evidence, because intentions and beliefs are taken into consideration in transactions. The proofs for this principle are very numerous, among them the Prophet’s (peace be upon him) cursing of the one who presses grapes for wine and the one for whom it is pressed. This curse is only deserved if it becomes clear to him that the buyer intends to use his money as a means to commit forbidden acts. This falls under the Almighty’s saying: {And cooperate not in sin and aggression.}

As Imam Ahmad mentioned, the Messenger of Allah (peace be upon him) forbade selling weapons during times of strife, and this is a pre-emptive measure to block the means to assist in sin.

If all the uses of an item are forbidden, then it is prohibited to sell it. If they are lawful, its sale is permissible by consensus. If they are mixed, then if the majority of the forbidden uses are intended, the sale is prohibited. This is an attachment to the prohibited category, because the existence of an intended forbidden use indicates that it has a share in the price, and a contract cannot be divided. Averting harm takes precedence over achieving benefits.

As for the ruling on supervising the construction of water games, the consideration is what the questioner predominantly believes about how these games will be used. If he predominantly believes that their common use will be in a permissible manner, then working on them is not forbidden. But if forbidden acts and unlawful use predominantly occur, then one should not work on them.