Do you have an answer about why Imam Ali(as) didn’t did not return the land of Fadak to the heirs of Fatima(as) when he became caliph?

Yes. He said I will not forcefully take that which was forcefully taken. Allah will be judge on Qiyama. Plus he couldn’t just use his power because people will say just because it's Khalid’s he took it not because Fadak belonged to Fatima (a)