What is the ruling of Islamic law regarding a wife remaining employed in another country with an excellent salary while refusing to return to her husband's country, where the salary is low, and the husband refuses to join her? Is this considered a form of nushuz (disobedience/rebellion) on her part?

If the wife travels for work with her husband's consent, and her work fulfills the Islamic legal conditions, then there is no blame upon her, and her money is her own, unless the husband stipulates a portion of it in exchange for his permission for her to work, in which case he is entitled to that portion due to the condition. However, if her travel is without the husband's consent, or if she refuses to return after he requests it, then she is sinful and disobedient (nashiz), and her right to maintenance, clothing, and housing is forfeited.