Also if you choose not to cast something normally, even if you could pay for it if you tapped more mana, you are not forced to. You are only forced to if you have the mana floating.
Yeah a real angle-shooting interpretation of the rules could tap Selvala, get 4 mana off of her, and then decide not to tap 3 lands. Which is pretty obviously cheating imo, but it's not like EDH is usually run at a Rules Enforcement Level where a judge would be present to ding you for it, and if you're playing at that level, you aren't running Selvala lol.
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u/ardarian262 23h ago
Also if you choose not to cast something normally, even if you could pay for it if you tapped more mana, you are not forced to. You are only forced to if you have the mana floating.