Why does it say "ki'Devar Hashem
Radak #1: Just like Hashem said that he will come to me, so he did. Then I knew that Hashem intended that I buy it 1 .
Radak #2: Chanam'el came based on Hashem's word [to Yirmeyah. Yirmeyah told others, who told him. Chanam'el said, when you buy it, I will know that it is Devar Hashem 2 .)
Malbim: Do not say that Yirmeyah was unsure about his Nevu'ah until Chanam'el came. Nevi'im had no Safek at the time of Nevu'ah. They knew clearly that it is from Hashem 3 ! This is a great principal of Nevu'ah. Rather, Yirmeyah was unsure of the Perush of his Nevu'ah. Perhaps "Chanam'el
Perhaps initially Chanam'el was unsure, for Yirmeyah's enemies often said Sheker in his name to discredit him. Buying it would be a proof, for Yirmeyah would not buy it on his own, since he was incarcerated, and he foretold that Galus is coming. (PF)
The first times that Hashem called Shmuel, Shmuel thought that it is Eli (Shmuel I, 3:4-8)! Malbim must discuss the content of the Nevu'ah, or after the first Nevu'ah. - PF)
Why does Yirmeyah profit? If he is the heir, even if he does not buy it, he will get it after Yovel! He will not benefit from it now, for Galus Bavel will start next year, and last 70 years! (PF)
