All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
According to the sound opinion, Sa‘y between Safa and Marwah is a pillar in both Hajj and ‘Umrah without which they are not valid. Hence, if you performed Hajj, be it Tamattu‘, Qiran or Ifrad, and did not do Sa‘y after Tawaaf al-Qudoom (the arrival Tawaaf), then you then have not yet ended your Ihram (ritual state of consecration) with the second (major) dissolution because the second dissolution takes place only after Tawaaf al-Ifadhah and Sa‘y between Safa and Marwah. This rulings applies regardless of whether this Hajj is for yourself or on behalf of someone else. Therefore, it is due on you to go to Makkah and complete the missing rounds of Sa‘y. As a precautionary measure, you should repeat Tawaaf and Sa‘y from the beginning in avoidance of the difference of opinion of those who regard it obligatory to do Tawaaf and Sa‘y consecutively and to observe consecutiveness in performing the rounds of Sa‘y as well. When you do so, you will have finished your Ihram; and if you are unable to go to Makkah, then you will be subject to the ruling of the muhsar (one who is prevented from completing the rituals due to a considerable reason), which is to finish your Ihram by sacrificing the hady (sacrificial animal). If you are still unable to do so, then ten fasts will be due on you, after which you will have finished your Ihram. That is the first opinion concerning Sa‘y.
The second opinion is that Sa‘y is an obligation rather than a pillar and that leaving it does not invalidate Hajj but requires slaughtering a sacrifice.
The first opinion is the preponderant one according to us.
Allaah Knows best.