During a Premier Draft match, I attacked with Fire Sage, spending no mana while attacking, and after damage was resolved, I was sure my opponent was not going to do anything, and then I activated Fire Sage's ability.

However, after combat ended, the damage resolution step was skipped, and the red mana added by the fire technique disappeared.

He was unable to use the Fire Sage's ability, which he should have been able to activate, which led to his loss in the match.