Rebuke the violent says:
Immediately after an attacker within 30 feet of you deals damage with an attack against a creature other than you, you can use your reaction to force the attacker to make a Wisdom saving throw. On a failed save, the attacker takes radiant damage equal to the damage it just dealt.
There are several situations where the damage the attacker rolls and the hp taken off differ. Rage, interception, deflect missile and resistances to poison etc.
In the situation where a thug shoots the paladin's monk ally for 10 points of piercing damage with the monk's deflect missile reducing that to 0 points. Would rebuke the violent deal 10 or 0 points of damage?
I did find a similar question which instead looks at when a creature is taken to 0hp and the damage was actually taken rather than actively being deflected or absorbed by resistance - especially given that in some cases the damage could be reduced to 0.