The Infamous Cruelclaw
The Infamous Cruelclaw ability not allowing for cards to be cast for their mana cost. Note:
"Whenever The Infamous Cruelclaw deals combat damage to a player, exile cards from the top of your library until you exile a nonland card. You may cast that card by discarding a card rather than paying its mana cost."
This wording suggests the card should be able to be played for its mana cost, as per normal. However this didn't happen, card was exiled.
Opponent also had The Infamous Cruelclaw in deck and the same thing happened to them.
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Rezzahan#77802 commented
The "may" is in regards to casting the spell or not. The cost has to be the alternative one, you do not get to cast it for its normal cost. So if you want to cast that card, you pay the stated cost, or you do not cast the spell. Those are your only options.