Can a spell with a buyback cost be countered after the buyback cost is paid?

Buyback is a keyword ability that lets you pay an additional cost when casting a spell. Paying the buyback cost happens during the casting process itself — specifically during the step where you pay costs (CR 601.2h). Once all costs are paid and the spell is on the stack, it is a legal target for counterspells.

When a spell with buyback is countered, it is put into its owner's graveyard as a normal countered spell (CR 701.5a). The buyback cost was already paid, but since the spell never resolved, the buyback effect — which would return the card to your hand instead of the graveyard — never triggers (CR 702.27a). You do not get the mana or resources spent on buyback returned to you.

This is because buyback's return-to-hand instruction is part of the spell's resolution. If the spell doesn't resolve, that instruction is simply never carried out. Countering a spell prevents all of its effects from occurring (CR 608.2b applies only to resolving spells).

Example: You cast Capsize with buyback, paying the full 6 mana. Your opponent responds with Counterspell. Capsize is countered and goes to your graveyard — you spent 6 mana but get nothing, not even the card back to your hand.

There is no conditional here: buyback spells are always vulnerable to counters after casting, regardless of whether the buyback cost was paid.