Would you aceppt that a player declare was non letal post mortem?

What I mean Is something like the party is in combat, the player make an attack an they killed a NPC he doesnt wanted to kill, then he declared It was non letal, would you allow It?

Pd: this happen to my in a game. I don't want to kill the NPCs to don't scalete conflict with certain factions. I ask the DM if my character with his passive medicina could know if I the enemy would survive a firebolt, he said yes, then I crític máximum damage. What very hilaruous in moment, the DM told me that magic damage couldn't be non letal, something I agree and don't complain.