It appears that the Rambam has a different way of understanding the conclusion of the gemara. In his Peirush Hamishnayos on Gittin, he writes as follows on the mishna which says that if one does ניסוך בשוגג he need not pay while במזיד he must pay damages:
"ומה שאמר המנסך אפשר זה על א' מב' דרכים וכו' והדרך השני שינסכהו לע"ז וידע שהיין אסור בזה הניסוך כשיהיה מזיק בנזק ולא ידע שהוא חייב סקילה כשמנסך אותו לע"ז, לפי שאם היה יודע ג"כ שהוא חייב סקילה לא היה חייב בתשלומים לפי שהעיקר אצלנו אין אדם מת ומשלם במו שביארנו בפ"ג מכתובות".
He says that the case of מנסך is that he knew that ניסוך is אסור and that he is causing damage [because the wine is forbidden] but didn't know that there is a חיוב סקילה, because if he would know about the חיוב סקילה he would be פטור. For a person cannot be חייב מיתה and pay as well according to the rule of קים ליה בדרבה מיניה. Therefore it [במזיד חייב] is talking about a case where he was שוגג on the act of ניסוך i.e. he didn't know that there is a חיוב סקילה, but מזיד on the damage - he knew that what he was doing is forbidden and damaging.
The commentaries had a LOT of trouble with this explanation. How can the Rambam say that if he is שוגג on the איסור ניסוך, then he must pay? He paskens in numerous places like תנא דבי חזקיה that if one performs an איסור בשוגג for which he would be חייב מיתה if done במזיד, then he is פטור from payment.
Also, how can he say that he would be פטור because of קים ליה בדר"מ [if he knew that there was an איסור סקילה]. In the gemara it says that there is no קלב"מ, because he already acquired it at the outset when he picked it up and the חיוב מיתה only transpired when he did the ניסוך. So how can the Rambam say that there is a פטור of קלב"מ when the gemara says that there isn't because the חיוב ממון took effect before the חיוב מיתה??
It seems that the Rambam explains that the reason Rav obligates for מנסך ממש [he actually made a libation to an idol and not that he mixed it with יין נסך as Shmuel held] is because he acquires it when he lifts it up [even though the lifting up is in order to pour it] and the mishna is talking about מנסך בשוגג. Rav holds that there is a distinction between the din of קלבדר"מ when the עבירה is done במזיד and the din of those who did an act בשוגג for which they would be חייב מיתה במזיד. This principle of Tosfos that קלב"מ effectively renders the מעשה as having been done by a different party is only במזיד but not בשוגג because שוגג was only included in the basic דין of קלב"מ, that there is no חיוב ממון for a מעשה for which one would be חייב מיתה if done במזיד, but not that the מעשה is "deactivated" and doesn't relate to the doer.
We see this from the Maharshal [cited in the Ktzos 28-1] who says that in a case where קלב"מ applies, one has to be לצאת ידי שמים [fulfill his Divine mandate] and pay and that תפיסה is effective [the victim may forcibly take the item back]. But that is only בשוגג but במזיד where the more severe punishment is actually carried out, there is no חיוב even לצאת ידי שמים and תפיסה is ineffective. The distinction between שוגג and מזיד can be explained as follows: בשוגג we don't say that it is as if someone else did it but rather it is just a פטור from the חיוב. There we say that there is an obligation לצאת ידי שמים. Whereas במזיד, when the more severe punishment is meted out, we view the act [that obligates payment] as having been done by a third party, so there is not even a חיוב לצאת ידי שמים. [When we carry out the more severe punishment it is like payment has been rendered because the bodily punishment encompasses the monetary payment. So there is no חיוב left which would mandate a rule of לצאת ידי שמים. Hence, he is completely off the hook].
Based on this we understand that there is a distinction between מזיד and שוגג even with respect to the lifting of the barrel, about which the gemara says מדאגביה קנייה - he is קונה at the time of the lifting. If the ניסוך is done במזיד [i.e. he knew that there is a חיוב סקילה], then the ENTIRE מעשה is divorced from him [as we saw in the part 1], even the part that is not actually מחייב מיתה and therefore he is not liable for the act of lifting up the barrel. Only בשוגג Rav holds that that he is חייב for the lifting [i.e. גזילה], because בשוגג it is not the entire act that is פטור but rather the חיוב מיתה proper is פוטר, meaning that for the very act for which he is חייב מיתה he cannot be חייב ממון. In that case, Rav said that he can only be exempted during the same stage [of איסור] and not for a different act at a earlier or later stage.
That is all according to Rav. But Shmuel argues and and says that we can't establish the mishna as saying that he must pay when he is מנסך ממש because of קים ליה בדר"מ. Based on our understanding Shmuel's reason is that he holds that we can't distinguish between שוגג and מזיד when it comes to קלב"מ. Just like with regard to מזיד, we say that it is as if someone else had done the ניסוך, so too שוגג because the rule of קלב"מ applies in the same way to both [not like the principle of the Maharshal]. It is the ENTIRE מעשה which creates the exemption. And therefore he holds that you can't say מדאגביה קנייה - that when he lifts up the barrel he acquires it and is considered a thief because ALL parts of the מעשה are exempt due to קלב"מ - even the part that is not מחייב מיתה. Thus, he is פטור even for the lifting.
To summarize - Rav holds that only if he does the ניסוך במזיד [knowing that there is an איסור סקילה] we say that the entire act doesn't relate to him - including the lifting. But if he was שוגג on the ניסוך [he knew that he is doing damage but thought that there is no סקילה], then he can be obligated to pay for the lifting of the barrel. Shmuel holds that even when he is שוגג on the ניסוך, he is totally exempt. So he had to come up with another explanation why he would have to pay [and so he said that it is talking about when he mixed it with יין נסך].
That is all according to Rav. But Shmuel argues and and says that we can't establish the mishna as saying that he must pay when he is מנסך ממש because of קים ליה בדר"מ. Based on our understanding Shmuel's reason is that he holds that we can't distinguish between שוגג and מזיד when it comes to קלב"מ. Just like with regard to מזיד, we say that it is as if someone else had done the ניסוך, so too שוגג because the rule of קלב"מ applies in the same way to both [not like the principle of the Maharshal]. It is the ENTIRE מעשה which creates the exemption. And therefore he holds that you can't say מדאגביה קנייה - that when he lifts up the barrel he acquires it and is considered a thief because ALL parts of the מעשה are exempt due to קלב"מ - even the part that is not מחייב מיתה. Thus, he is פטור even for the lifting.
To summarize - Rav holds that only if he does the ניסוך במזיד [knowing that there is an איסור סקילה] we say that the entire act doesn't relate to him - including the lifting. But if he was שוגג on the ניסוך [he knew that he is doing damage but thought that there is no סקילה], then he can be obligated to pay for the lifting of the barrel. Shmuel holds that even when he is שוגג on the ניסוך, he is totally exempt. So he had to come up with another explanation why he would have to pay [and so he said that it is talking about when he mixed it with יין נסך].
According to this, the words of the Rambam in the Peirush Hamishnayos and the gemara complement each other. The root of the obligation of the מנסך is because he is a גזלן when he lifts up the barrel of wine, as the gemara says, because we don't say that the lifting is for the sake of ניסוך. But this is all בשוגג, as the Rambam explained in his Peirush Hamishnayos. When done במזיד [i.e. he knows that there is a חיוב סקילה], since קים ליה בדר"מ and we carry out the more severe punishment, we go even further and say that the act obligating money was done by another person. That would include then all parts of the מעשה including the lifting of the barrel in order to do ניסוך. Only בשוגג where we don''t have this earth shattering chiddush that the entire act is effectively deactivated from him, we are able to view the lifting independently and obligate him. So now we understand the Rambam who establishes the case where he must pay as talking about שוגג [on the חיוב סקילה but מזיד on the damage]. And we are no longer troubled by the fact that the Rambam paskened like תנא דבי חזקיה that חייבי כריתות שוגגין are פטור from payment, so he should be פטור despite the fact that he is שוגג. The answer is that he is חייב for the act of lifting the barrel. שוגג only makes him פטור when he was שוגג on the same act that would be מחייב מיתה if done במזיד. If the ניסוך would have been done במזיד, then he would also be off the hook for the lifting, because מזיד detaches the entire מעשה from the sinner vis a vis the monetary obligation.
