Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.

First we cannot confirm any punishment to any bad deed done before. Although the idea is possible, we have no right to connect a hardship like illness to a sin that happened before. Only Allah the Almighty and the Prophet (peace be upon him) can do that.

Second, any repentance has 4 conditions to it: remorse, stop, intend not to repeat and clear your issues with person you harmed.

Since you did not specify what the mistake you did was, you can go ask the person for forgiveness and return anything you took from him if that was the case.