is rebuttal a delayed trigger for paying the additional power

No — Rebuttal's [A] payment is not a delayed trigger. It's a cost within instructions, paid during Rebuttal's resolution (Rule 204.3.b), just like Bullet Time's power payment.

What that means:

• Rebuttal's only cost to play is its printed 1 Energy + 1 Power. The [A] is an optional cost you decide and pay when Rebuttal resolves, not when you finalize it.
• Your opponent can react to Rebuttal on the chain before you pay — and if they counter it, you pay nothing.
• If you do pay [A], you gain control of the chosen spell and remake its choices; if you don't, you counter it instead. (Confirmed by the Vendetta FAQ: "they pay [A] during resolution of Rebuttal.")

Note: the FAQ on Rampage clarifies you can't use Rebuttal to pay another spell's unpaid optional additional costs — those were "as you play" costs, already decided and excluded from remade choices (Rules 752.2, 755). Rebuttal's own [A] is different: it's built into its effect text.