If an opponent is attacking me and I stun the attacking unit, the units recall. Opponent readies the unit and attacks the same battlefield, does the opponent's unit deal combat damage?

Yes, the opponent's unit will deal combat damage, provided it is no longer stunned when the Combat Damage Step occurs.

Here is the breakdown of why:

1. Stun Status: According to Rule 410.1.b, "A Stunned Unit does not contribute its might to damage in combat." However, Rule 410.1.a.2 states that "Stunned Units lose the Stunned status at the beginning of the next Ending Step."
2. Readying: If the opponent uses an effect to "ready" their unit, they are likely removing the exhausted status, but this does not automatically remove the "Stunned" status unless the card specifically says so.
3. Combat Damage: If the unit is still stunned when the combat reaches the Combat Damage Step, it will not contribute its might to the damage calculation. If the unit is no longer stunned (either because the turn ended and a new one began, or a specific effect removed the stun), it will contribute its might to the combat damage as normal.

In summary: The unit's ability to deal damage depends entirely on whether it is currently Stunned at the moment combat damage is resolved. If it is not stunned, it deals damage based on its current might.