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Last turn, player A casted a Pact spell, and then a copy of Mindslaver's ability, controlled by player B, resolved while targeting player A. ​ In player A's untap step, player A untapped 5 basic lands of the type that corresponds to the Pact spell's color. ​ Tapping those lands for mana is the only way to tap those lands, and paying for the Pact spell's delayed triggered ability is the only way for player A to spend the mana which those lands can produce.

Can player B nonetheless choose to have player A not pay for that triggered ability?

The Pact spells do not make the payment optional, but there is precedent for players not being forced to pay costs. ​ (See the two sentences with the word "forced" in that link.)

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Player B can choose to not tap the lands, and thus be unable to pay the cost.

Rule 117.3c says

Activating mana abilities is not mandatory, even if paying a cost is.

So, when the ability resolves, player B can choose not to tap the lands, then player A fails to pay the cost because they do not have enough mana.

  • I believe that even when you tap the land for mana and the mana is in the manapool and the pact's trigger resolves player B can choose to not pay for the trigger, but I'm not sure. Can you clarify this? – Ivo Beckers Sep 11 '15 at 9:29
  • @NoBeckers: ​ For that, you should read the cited rule's example. ​ ​ ​ – user13741 Sep 11 '15 at 10:54
  • I guess the rule's example is not exactly the same situation but I believe indeed that I was wrong and that you do have to spend mana if you have it – Ivo Beckers Sep 11 '15 at 11:11
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    The first sentence is true for the situation described in the question, but false in general. You can't choose to not pay the cost if you have the mana in your pool. I suggest revising to "Player B can choose to not tap the lands, and thus be unable to pay the cost." – Rainbolt Sep 11 '15 at 14:03
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    @NoBeckers, No, you'd have to pay it. You must follow the instructions of effects, the effect tells you to pay. If you can pay the cost (e.g. if the cost consists of mana and you have the mana), you have to pay it. You aren't instructed to produce mana, so you aren't obligated to do that, as codified by 117.3c. If it said "you may pay ...", then you'd have a choice. – ikegami Sep 11 '15 at 14:40

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