بسم الله الرحمن الرحيم
The ruling regarding fardh Hajj-e-Badal is that if hajj was fardh on someone and he found the time of hajj but could not perform it while he was capable, and later on he turned helpless and unable to perform hajj. In such case it is fardh on him to send someone on his behalf for Hajj-e-Badal or make a will that hajj is performed on his behalf (Jawahir al-Fiqh 4/205, published Zakaria Deoband).
We are quoting the summary which Hadhrat Maulana Mufti Shafi (RA) has mentioned after counting the conditions of fardh Hajj-e-Badal: “Two words shall occur repeatedly in its conditions. One is Aamir and the second one is Mamur. It is necessary to understand their meaning. Aamir means the person who is sending someone for hajj. Mamur means the person who is doing hajj on behalf of someone. The conditions are as follows:
Four conditions are related to the person who is sending for Hajj-e-Badal. (1) He is Muslim. Hajj is fardh on him and he is unable to perform it. (2) His excuse is perpetual. (3) He became unable before sending someone for Hajj-e-Badal. (4) To appoint someone for Hajj-e-Badal or make a will for it. While four conditions are related to the Mamur: (5) He is Muslim. (6) He is sane. (7) If he is not adult then should be at least adolescent. (8) He will not take any compensation for hajj. Rest conditions are related to the practices of Hajj. (9) In doing Hajj-e-Badal most of the expenses shall be made by the Aamir, if he includes something from himself then there is no harm. (10) Most of the way shall be covered by vehicle; if he travels on foot then the hajj of Aamir shall not be valid. (11) The hajj should be started from the country of Aamir. (12) He should not render hajj invalid. (13) The intention of hajj should be made on behalf of Aamir while wearing ihram. (14) He should not miss it. (15) He should not oppose the Aamir in any of his conditions. There are five more conditions that belong to condition No 15, they are not separate conditions.” (Jawahi al-Fiqh: 4/213-214, Hajj-e-Badal, published Zakaria Deoband). In order to know more detail see the book Jawahir al-Fiqh 4L203-228, Anwar-e-Manasik, p 541 etc.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.