The skeptics claim that Ali (a.s.) should have taken back Fadak if it was indeed the property of Hazrat Fatima Zahra (s.a.). By leaving Fadak as it is, Ali (a.s.) has admitted that the property did not belong to Hazrat Fatima Zahra (s.a.).
The hollowness of such claims are exposed in many reports on Fadak, one of which is documented here:
Imam Sadiq (a.s.) informs us why Ameerul Momineen (a.s.) did not take back Fadak on assuming caliphate.
He narrates: Ali (a.s.) did not take it back because the oppressor and the oppressed (Hazrat Fatima Zahra s.a.) had already set forth towards Allah, the Almighty. Allah had rewarded the oppressed (for patience) and had punished the oppressor (for usurping). Then Ali (a.s.) did not wish to take back something for which Allah had already punished the oppressor and had rewarded the oppressed.
- Elal al-Sharae p 155 under Chapter of ‘Why Ameerul Momineen (a.s.) left Fadak as it is on assuming caliphate’
- Awalim al-Uloom v 11 p 766
Clearly, there was a a higher purpose in forsaking Fadak. Ali (a.s.) out of respect for the divine reward and punishment, considered it more prudent to leave it as it is.