Is it a sin or a reward for a person who, upon confronting a vendor who mockingly and scornfully held up a piece of underwear his sister had bought in front of others, broke the vendor’s hand? And what should he do if he has committed a sin?

If the man mocked the questioner but did not assault her honor, then she has no right to assault him and break his hand. She must repent to Allah, seek his forgiveness, and pay what is necessary for breaking his hand, as determined by the Sharia court. If the man forgives her or a reconciliation is reached, that is permissible. If the mockery was an assault on honor, then repelling the aggressor should be done with the easiest means available, then the next easiest.