If I were to cast Dark Deal with a Waste Not on the field and gained mana, would I be able to use that mana to pay the madness cost of From Under The Floorboards, one of the cards I discarded from the same Dark Deal? Or do I not get the mana until after the opportunity to pay madness costs has passed?
You'll get the mana after, unfortunately. Dark Deal would resolve and you both would cycle your hand, then you'd get the mana. Your old madness cards will be gone by then.
You can use that mana. You discard it, then it goes to exile. This triggers Waste Not and adds mana to your pool. You may then resolve Madness with it.
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Madness is a static ability and a triggered ability. The static ability replaces where the discarded card goes to (exile, instead of graveyard), and then the triggered ability let's you cast it.
Since Waste Not and the Madness trigger are both triggered abilities you control going on the stack at the same time, you choose which order they resolve in.
Since Waste Not and the Madness trigger are both triggered abilities you control going on the stack at the same time, you choose which order they resolve in.
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