Do I still need to pay a power if I play Void Assault on my unit moving to Sandswept Tomb?

It depends on where the friendly unit is when you play Void Assault, not where it ends up.

Sandswept Tomb's discount only applies to spells that choose a friendly unit that is already "here" at the time you determine the spell's cost (Rule 356.4). So if Void Assault is moving your unit to Sandswept Tomb from elsewhere, the unit isn't there yet — no discount applies, and you still pay the 1 Power (plus 2 Energy). If the friendly unit is already at Sandswept Tomb when you play Void Assault, the [A] discount reduces Void Assault's Power cost from 1 to 0, and you pay only the 2 Energy (same principle as FAQ #11846).

Note the discount doesn't care about the enemy unit you also choose — qualifying for it only requires at least one chosen friendly unit at Sandswept Tomb.