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According to Lama Je Tsongkhapa, actions done in a dream are actions done but not accumulated. On the other hand, setting the intention to kill someone and not doing it is accumulated but not done.
A karma that is done and not accumulated, or accumulated but not done, is a "karma whose result you not definitely but only possibly experience" according to Tsongkhapa. In addition, purifying an action that is done and accumulated, by way of the four opponent powers, transforms it into an action that is done and not accumulated.
In the Lam Rim Chen Mo, volume 1, Je Tsongkhapa says:
[Asanga's] Compendium of Determinations presents four permutations between tween karma done and karma accumulated. Killing that is karma which you have done but not accumulated is seen in the following cases: that done unknowingly, that done in a dream, that not done intentionally, [etc]
In the same text, he cites the Levels of Yogic Deeds:
Karma that you have accumulated is that not included among the following ten types of actions: actions done in dreams...
Upvote:2
I think it's listed as a point of controversy in the Abhidhamma -- Kv 22.6:
Controverted Point: That all dream-consciousness is ethically neutral.
22.6.1 TheravΔdin: You admit, do you not, that a dreamer may (in dreams) commit murder, theft, etc.? How then can you call such consciousness ethically neutral?
22.6.2 UttarapΔthakas: If I am wrong, was it not said by the Exalted One that dream-consciousness was negligible? If so, my proposition holds good.
I think the (TheravΔdin) argument is that intention (e.g. intention to murder) may be present, therefore it's not ethically neutral.
I think the counter-argument is that there's no material result, so if it's not neutral it's negligible.
See also the Vinaya -- that defines offences (e.g. killing) in some detail or legalistically (i.e. in more detail than the suttas do) -- and there, there are various (several) conditions which must be met for the offence of e.g. "killing" to have occurred, one of which is that somebody has to die -- and in a dream, nobody is really killed, so that wouldn't be counted (and punished) as the offence of killing.
The Abhidhamma is considered part of the Pali canon, even if it isn't all quotes from the Buddha.
In Tsongkhapa's Praise for Dependent Relativity it is mentioned that the karma of killing in a dream was a question answered by the Buddha, but no sources are provided.
So far as I known, the commentary to the Vinaya pronounces the non-punishment of sexual actions performed while dreaming (while also saying it's better not to).
It's not exacty right to equate the working of kamma with the rules of the Vinaya, however that may be the best we have -- because the Vinaya is defined in detail whereas the suttas don't describe kamma in every detail -- even say that the precise workings of kamma is known to the Buddha but is unconjecturable -- and, the Vinaya was defined by the Buddha.