פירוש על קידושין 51:3

In Mishnah Shevuoth 6:5 we learned that people do not take oaths over land. For instance, if Reuven claims that Shimon owes him land, and Shimon admits to part of the claim, he need not swear that he does not owe him the rest, as he would were Reuven to claim that Shimon owes him money or animals. However, if Shimon needs to take an oath over movable property and land, since he must take an oath over the movable property he must also take an oath over the land. This could happen if Reuven claims that Shimon owes him a piece of land and a 100 sheep. If Shimon admits that half of the land is Reuven’s and half of the sheep, he must take an oath over both the land and the sheep which he claims not to owe.