Is failing to fulfill a marriage to a girl because one is forced to borrow a large sum for the mother's treatment considered a breach of promise before God and the girl, and does the mother's treatment take precedence over marriage?

Treating the mother is a priority and an obligation. As for fulfilling a promise, scholars have differed regarding its ruling, some deeming it recommended, others obligatory, with the detailed view that fulfilling it becomes obligatory if not fulfilling it results in harm to the promised party. If it is possible to combine both matters, that is better. Otherwise, treating the mother takes precedence, and there is no sin in breaking a promise when one is unable to fulfill it. We also advise saying: "Allah Almighty willed" instead of: "Fate willed."