I think I am losing everyone with this topic, so let me finish it off. Ritva asked: according to Rashi that a lav hanitak l’aseh is violated up front but punishment is pushed off until the aseh is negated, why do we apply kim lei b’derabbah minei to the moment the aseh is violated? The lav causing malkos, not the aseh, should have to be simultaneous with the chiyuv misah to exempt the doer from malkos?
R’ Shimon Shkop (Sha’arei Yosher 1:3) draws the inevitable conclusion – according to Rashi, kim lei b’derabbah minei does not require that issurim be simultaneous, it requires that the trigger of two punishments be simultaneous. Since without fulfilling the condition of the anusa being eliminated and killed (in the case discussed in the previous post) there is no punishment for divorce, it is meeting that condition simultaneous with the act of murder which effects klb”m.
This helps answer the Rambam (see previous post) as well. Once a cow is borrowed, the borrower is liable to return or replace the cow from that moment, which is why even if the borrower dies, his estate is liable. If so, why is a borrower who slaughters a cow on Shabbos exempt from payment because kam lei b’derabbah minei – the obligation for repayment occurrs when he borrowed the animal, not simultaneous with Shabbos? The answer is (see Ktzos 341) that when it comes to klb”m we don’t look at when the potential obligation of repayment occurred, we look at when the actual obligation of repayment is triggered. It is as if the obligation to pay exists on condition (tnai)– the lien is in effect from the moment the obligation is set, but klb”m applies to themoment the condition effecting payment is fulfilled.
For more on klb”m and tnai, see the Divrei Yechezkel who has a nice roundup of sources. These type topics are very hard to break into bite-size chunks and render into English. Sorry if the ride was a bit rought.