Hello, I recently reread the 601.2h (full text at the end of this post) and realized I didn't know what it really entails.
1) Suppose I control a banisher priest exiling a vanilla creature card I own (perhaps I donated it before playing the priest). I start casting Torgaar, Famine Incarnate, announce my intention to sacrifice 2 creatures, tap my lands for mana, and basically have "2BB, sacrifice 2 creatures" to pay.
Can I pay this cost by sacrificing my priest, immediately bringing back my other exiled creature, then sacrifice that one as well? or must I actually sacrifice two creatures simultaneously as it would count as two "partial payments" of some undivisible "sacrifice 2 creatures" part of the cost?
I'm casting ruthless disposal (targeting two irrelevant creatures an opponent controls).
While paying its cost, I tried to sacrifice a creature (the animated banishing light) before discarding a card, which immediately brought back heightened awareness exiling in turn my hand as it entered the battlefield, preventing me from legally completing the spell's payment. Yet I already sent banishing light to the bottom of my library because of wheel of sun and moon which seems to prevent the sacrifice from being reversed when handling illegal actions (according to 721.1).
Did I already break the rules by not sacrificing a creature and discarding a card simultaneously (c.f. question 1)? Perhaps I did something else wrong? If not, how do I proceed from here?
3) In general, am I allowed to start paying a cost only to change my mind before I complete its payment and undo the whole casting process? If yes, what happens if my already completed payments are somehow irreversible?
(e.g. previous question, or casting invigorate while my opponent controls nefarious lich and sphere of resistance, assuming I "have my opponent gain 3 life" first and change my mind before paying 1)
quoted rules (from battlebond complete rules, June 8, 2018):
601.2h The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can’t be paid.
Example: You cast Altar’s Reap, which costs 1B and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost 1 less to cast. Because a spell’s total cost is “locked in” before payments are actually made, you pay B, not 1B, even though you’re sacrificing the Familiar.
721.1. If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was casting a spell, the spell returns to the zone it came from. The player may also reverse any legal mana abilities activated while making the illegal play, unless mana from them or from any triggered mana abilities they triggered was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, caused a library to be shuffled, or caused cards from a library to be revealed.
1) The cost of sacrifice two creatures can't be divided into sacrifice a creature then sacrifice another creature, you pay each portion of the cost completely but in any order you want.
2) This is even simpler. Heightened Awareness's ability that causes you to discard you hand is a trigger, meaning it doesn't just happen. It triggers, waits to go on the stack then eventually resolves. You will have payed the whole cost before the trigger makes it to the stack.
3) You can not begin casting a spell then "change your mind". The only reason you can uncast a spell is if something proposed action is illegal. So if you announce Invigorate in the scenario, begin resolving the payments only to find that the action is impossible; because there are no legal creatures in play, lets assume they all have hexproof or progreen or you didn't notice the sphere and can't pay the 1. You simply call a judge and they will sort it out. If they found that you did this on purpose, then it is cheating and you will be DQd. Otherwise they will decided the apporpriate way to handle the event.
1. Torgaar says "you may sacrifice any number of creatures". If you pay for Torgaar by sacrificing two creatures, one of which is Banisher Priest, both creatures will be sacrificed simultaneously. Thus, the card exiled with Banisher Priest will return to the battlefield only after you sacrifice both creatures (C.R. 610.3). This is because a single action verb, "sacrifice", is used (see also C.R. 608.2c, 608.2f).
2. Here, Ruthless Disposal says "discard a card and sacrifice a creature". If you sacrifice Banishing Light first (and put it into your library, assuming you own Banishing Light and Banishing Light isn't a token) (costs to cast a spell are paid in any order under C.R. 601.2h, as you pointed out in comment 1), the card exiled with Banisher Priest (here, Heightened Awareness) will return to the battlefield immediately after (C.R. 610.3), so before you discard a card. Since you do so, you discard your hand as Heightened Awareness enters the battlefield (note that its first ability is not a triggered ability; it happens upon Heightened Awareness entering the battlefield [C.R. 603.1, 614.1c, 614.12, 400.6]). This is because two instances of an action verb — "discard" and "sacrifice" — are used (see also C.R. 608.2c, 608.2f). However, you can no longer discard any cards, so you can't discard a card to pay for Ruthless Disposal, so the action of casting the spell is reversed (C.R. 723.1). However, the action of sacrificing Banisher Priest "moved [a] car[d] to a library", so at least that action can't be reversed under C.R. 723.1.
3. In general, if a player has done an illegal action (for example, proposing an illegal shortcut or choosing an illegal target for a spell while casting it), the action is reversed (C.R. 723.1). In general, however, if the actions are not illegal, they are not reversed and the player must continue with the actions. See also this thread. Note that in the case of Invigorate and Nefarious Lich, it is still possible to pay the cost of having an opponent gain 3 life, even though it's replaced with something else due to Nefarious Lich (C.R. 118.11). Compare Nefarious Lich with Everlasting Torment. In a sanctioned tournament, a player is "not usually allowed to take back an action that has been communicated to their opponent, either verbally or physically" (M.T.R. 4.8). An exception is if a judge decides with certainty that a player "has not gained any information since taking [an unintended] action" and decides to "allow that player to change their mind" (M.T.R. 4.8).
EDIT (Oct. 2, 2018): Edited answer 3 to conform to a new M.T.R. section added with Guilds of Ravnica.
EDIT (Jan. 25, 2019; Apr. 28, 2019): Correctness edit.
EDIT (Nov. 29, 2020): Some rules were renumbered in the meantime. Add rule citations.
With the update for Adventures in the Forgotten Realms, the rules changed on how costs of a spell or ability are paid.
Now, while paying the costs of a spell or ability, you must first "pay all costs that do not involve random elements or moving objects from hidden zones to public zones in any order", then "all remaining costs in any order" (C.R. 601.2h). And discarding a card involves moving a card from the hand (a hidden zone) to the graveyard (a public zone) (C.R. 400.1, 701.8a). Thus, in general, to pay for Ruthless Disposal, you must first sacrifice a creature and pay the mana cost in any order, then you must discard a card. You are no longer allowed to, say, discard a card, then sacrifice a creature, then pay the mana cost.