In the case where the land had been modified by the digging of ditches, round wells and pits or caves: I follow the rationale where according to Rashi this is a destructive change.
Still it is not hard to imagine a scenario where these same activities would be considered an improvement and even (as we find later in Bava Basra) the basis for a chazakah! So...why don't we say that here? Or does the case refer ONLY to those circumstances where damage was done?
The Gemara is referring to where there is a liability, since we are looking for a case where there is an admission in the case of land owed without there being "Heilach." If the land is improved, obviously there is nothing to compensate.