Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Yes, Hajj is valid if Sa`i is performed after Farewell Tawaf, especially if done out of ignorance. The additional Sa`i does not invalidate the Hajj.
Answering your question, the Fatwa Center at IslamWeb, states:
Was My Hajj Accepted If I Did Sa`i After Farewell Tawaf?
Your Hajj is valid as long as you performed all its pillars and obligations correctly. Performing an additional (unintended) Sa`i during Hajj does not invalidate it, even though performing an optional Sa`i is not legislated in this context.
Are You Sinful for Doing Sa`i after the Farewell Tawaf?
You are not sinful for performing Sa`i after the Farewell Tawaf out of ignorance. This act does not contradict the requirement that the last ritual performed before leaving Makkah should be Tawaf of the Sacred House.
What Can We Learn from `Aishah’s Example?
An example supporting this is the case of `Aishah (may Allah be pleased with her). After performing Hajj, she did `Umrah with her brother `Abd Ar-Rahman, during which she performed Tawaf and Sa`i. It was not reported that she performed a separate Farewell Tawaf afterward. This indicates that the Tawaf she performed for her `Umrah apparently sufficed for both the Tawaf al-Ifadah and the Farewell Tawaf.
Scholarly Insight: Ibn Hajar’s Commentary on This Issue
Ibn Hajar (may Allah have mercy on him) commented: “The story of `Aishah indicates that if Sa`i takes place after Tawaf Al-Ifadah—assuming that Tawaf al-Ifadah also suffices as the Farewell Tawaf—then the Sa`i performed between that Tawaf and departure from Makkah does not affect the validity of the previous Tawaf.”
The difference between your case and that of `Aishah is that her Sa`i was legislated and valid under Islamic law, while your Sa`i was not legislated in that context. However, it does not affect the validity of your Hajj because you acted out of ignorance, thinking the practice was valid. Moreover, the nature of the act (i.e., Sa`i) in both cases remains the same.
Almighty Allah knows best.
Source: Islamweb.net with editorial changes.