The Qetzos asks how these two cases differ.
The basic difference is that the slave’s natural state is to have a power of domain. His being a slave is a monei’ah, an impediment, holding back that natural state from expression. The writ is therefore hasaras hamonei’ah removing that impediment. The field, however, is not already part of the buyer’s domain. Rather, the contract is a sibah, a cause.
Rav Amiel explains that causes must precede their effects. Therefore, the field must be acquired before it can be used as domain for receiving acquisitions. However, a hasaras hamonei’ah need not be earlier than the effect. Implied in this explanation is that the sibah is already acting even during the suppression by a monei’ah, thereby preserving the necessary time sequence.
Halachicly, a sibah and a monei’ah are very different things. It’s not just that a monei’ah is a negative sibah, a cause for the opposite state.
Until a person is a goseis (expected to die within 72 hours), there is no immediate cause for death. Anything that would cause death at that point would be new, in the nature of a sibah. However, for a goseis, the cause is already there. If medicine is presenting death, it’s a monei’ah. Once we conclude a monei’ah is involved there are two further possibilities: either hasaras hamonei’ah (“pulling the plug”), or one can refuse to introduce the monei’ah to begin with; active or passive.
The last option is by far the most often permissable.
To look at physical cases:
Removing Terry Shialvo’s feeding tube was a sibah for her death. She would not have otherwise died, and in fact died more than 72 hours later even without drink or food.
Removing artificial respiration from a goseis would be haras hamonei’ah.
However, once the machine is disconnected, for example to change a filter, to reconnect it would be to refuse the monei’ah to begin with. Although it’s not the removal of something stopping the death, one is allowing the actual cause to run its course rather than initiating the causal sequence. And, as we saw, that’s a halachicly different species.