When you play nashor with harrowing do you pay the baron cost on finalization or resolution? If harrowing gets countered do you lose 2 or 5 power in this case ?

You pay Baron's cost during the play itself, at step 4 "Pay Costs" (Rule 357) — before he's finalized, not at his resolution. Harrowing waives only his Energy cost (356.1.b.2), so his 3 Chaos power is paid as you play him; because units resolve immediately with no priority pass, your opponent never gets a window on the unit play at all — their only window is The Harrowing while it's still on the chain.

If The Harrowing is countered, you lose 2 power, not 5. The counter has to happen before Harrowing resolves, so Baron is never played and his 3 power is never spent — you're only out Harrowing's own 6 energy + 2 Chaos power, and countered costs aren't refunded (425.1.c). For reference, a fully resolved Harrowing→Baron line costs 5 power total (2 for Harrowing + 3 for Baron).

Worth noting: "counter a spell" effects can't hit Baron himself anyway, since he's a unit, not a spell — only Harrowing is a legal counter target here.