Many hadiths are narrated from our Prophet (p.b.u.h.) about the importance and performance of Hajj but there no saheeh ones relating that old or sick people, who can afford to it, have to send a representative. In case there does not exist an ayat or saheeh hadith related to an Islamic issue, we cannot suppose a duty as fard structuring our judgement upon comments. Because fard is a duty, which one’s fulfillment is required by means of certain and specific evidence.
Hanafi, Shafi, Shee, Dhahiri and Zaidi mazhabs primarily consider economical conditions and suppose sending representative as fard for old or sick people as long as they can afford. According to most Maliki ulama, health precedes wealth and they suppose Hajj as not fard for too old or sick people.
Abu Hanifah, Sarahse and Ibn-ul Humam however, consider health together with wealth and claim that a person who cannot afford by either of the means is not responsible for fulfillment of Hajj.
Imam Muhammad thinks that repsentative himself is supposed to have accomplished the duty of Hajj and the person who pays for it, gains sawab as much as of Hajj.
Ulama has consensus that it is wajib for the inheritors of a devisor, to send a representative to Hajj paying him from the inheritance of the devisor , who could not perform Hajj before death but willed to have a representative sent .
We consider that:
If a Muslim did not fulfill the duty of Hajj as he/she used to have the necessary conditions to perform it, but does not have anymore; then he or she cannot skip the duty by sending a representative only. That Muslim should rather repent from Allah for the liability of Hajj he/she is imposed and do the charity as much as Hajj expenses, than sending a representative to Hajj. Charity would be possible when he/she can still afford to it.