When a card says 'pay 2 runes', can I first exhaust the runes (spend them), then pay the runes that I've exhausted?

Yes, you can do this. In Riftbound, you can exhaust runes to add energy to your pool, and then recycle those same runes to pay for other costs (like a "pay 2 runes" requirement) in the same turn.

According to the game's mechanics and FAQs:

• Exhausting for Energy: When you exhaust a rune, it adds 1 energy to your resource pool. This energy remains available in your pool until the end of your turn.
• Recycling for Power/Costs: Recycling a rune is a separate action that adds domain-specific power to your pool or fulfills a specific "pay X runes" cost.
• The Sequence: You can exhaust a rune to generate energy, and then recycle that same rune to pay for a cost. The rune is removed from the board when recycled, but the energy it generated remains in your pool to be spent later.

Important Note: You must exhaust the rune before you recycle it if you want to get both the energy and the power/cost payment from it. If you recycle it first, the rune is gone and you cannot exhaust it.

As noted in the official FAQs:

"A single rune can provide both 1 energy (by exhausting) and 1 power (by recycling) in the same turn... You must exhaust the rune BEFORE recycling it to get both resources; if you recycle first, you only get the power."