To fully answer your question, no, you cannot sacrifice a creature after it has been dealt lethal combat damage. You can sacrifice it right before combat damage is dealt, but it will not deal combat damage to the creature it was blocking, nor will the blocked creature deal damage unless it has trample and vice versa.

What happens when you block a creature with double strike?

A card with double strike will assign combat damage in the first combat damage step. So if you attack or block with a creature that has double strike, that card will deal damage first and will not take into consideration any damage being dealt by the opposing creature.

What does double strike do to a creature?

Double Strike is a static ability that creates an additional combat damage step (similar to first strike ), but lets the creature deal combat damage again in the second step. A keyword ability that lets a creature deal its combat damage twice. See rule 702.4, “Double Strike.”

What happens at the end of combat damage step?

The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.

When does combat damage start in step 510?

510.4 If at least one attacking or blocking creature has first strike (see rule 702.7) or double strike (see rule 702.4) as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike.

Which is first strike or double strike in CR 702?

(See rule 510, “Combat Damage Step.”) 702.4b If at least one attacking or blocking creature has first strike (see rule 702.7) or double strike as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike.