骨たかりの守銭奴

If you discard a card as a cost to cast a spell or activate an ability, this creature’s triggered ability resolves before that spell or ability but after you’ve chosen targets for it. If you discard a card while a spell or ability is resolving, the triggered ability resolves after you’ve finished resolving that spell or ability.

This creature’s abilities are triggered abilities, not activated abilities. They don’t allow you to discard a card whenever you want; rather, you need some other way of discarding a card (such as cycling).

This creature’s second ability isn’t a mana ability. It uses the stack and can be responded to. Notably, this means that you can’t pay a cost of “{2}, Discard a card” by discarding a land card and using the {B}{B} produced to pay for the {2}.