פירוש על קידושין 41:6

Consecrating an ancestral field is both similar and different to the case of selling an ancestral field and selling a house in a walled city. Since there is something different from both, we cannot use either one to prove that one cannot borrow and redeem or redeem half of a consecrated ancestral field. But together, the commonality can be used to prove that just as in the other two cases one cannot borrow and redeem or redeem half, so too one cannot borrow and redeem or redeem half of a consecrated ancestral field.
We should note that this is a common midrashic technique—it is easier to learn one unknown from two knowns then from one known.