Is it permissible to work as a fundraiser for a non-Islamic charitable organization, and is the earned salary lawful, and is it permissible to encourage Muslims to donate to it, and will one receive reward and merit for doing so?

It is permissible to work with non-Muslim associations if their work is limited to charitable relief efforts, and the salary received from them is lawful. It is also permissible to call upon Muslims to donate to them from general charity funds, as giving charity to non-Muslims is a form of righteousness that Allah has permitted. As for receiving reward and recompense, it depends on the intention of the worker. If he has a good intention and aims to benefit the poor and needy, then he will have a reward from Allah. However, if the association has goals other than relief work, such as proselytization, spreading deviant ideologies, or warring against Muslims, then it is not permissible to work with them, and the salary would then be unlawful.