Is zakat obligatory upon the questioner's father for lands sold more than twenty years ago, or for land sold recently where he was hesitant to sell it, and how is it to be paid given his inability to recall the value of some of them or whether he had previously paid zakat on them, especially since he has partners who desire their share from the sale?

Zakat (definition) is obligatory on the land your father bought with the intention of trade, due to the generality of the evidence for the obligation of Zakat on trade goods. Zakat is obligatory on its value when a year has passed since the original capital was acquired. He must pay Zakat for every year it remained in his possession. If a long time has passed, the obligatory amount should be estimated with the help of experts. However, if he did not buy it with the intention of trading, then no Zakat is due on it.