Was the death of the wife, which was preceded by marital disputes and financial expenditure on her treatment, due to the husband's neglect of her and his failure to send sufficient money, or were there other reasons?

We ask Allah to encompass your wife with His mercy and to grant you patience, for life and death are in Allah's hands, and no one possesses the power to benefit or harm except by Allah's permission. The Sharia has commanded excellence in everything, and excellence towards one's wife in particular. Therefore, any shortcoming in fulfilling the wife's rights by cutting off communication with her and not spending on her is a sin that necessitates seeking forgiveness, for there is no obedience to a creature in disobedience to the Creator. However, this shortcoming is not a direct cause of her death. Our advice to you is to frequently seek forgiveness for her, give charity on her behalf, and honor and be good to her family.