Is it permissible for a man working in a charitable organization, to whom people have given money to be spent on charitable causes, to give this money to his poor fiancée so she can use it to prepare for their marriage?

If a man is an agent for donors in delivering money to a charity, then he must deliver it to the charity only, and the charity determines its expenditures. However, if he is an agent for the donors in spending money on charitable causes, and they have not specified particular expenditures, then there is no objection to him giving some of it to his poor fiancée.