Assalaamu alaykum. If I have a verbal fight one day in the street with an unknown person whom I have never met before, and then some other day, I get to meet him for the second time, is it a must to at least greet him with the Salaam? Can I not refrain from talking to him at all?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. {C}

It is confirmed that the Prophet said, "It is not lawful for the Muslim to desert his brother for more than three; they meet, so this one turns away and the other turns away, and the better one of the two is the one who greets the other first." [Al-Bukhaari and Muslim]

Therefore, it is not permissible for you to intentionally refrain from greeting this person for more than three nights, regardless of whether you knew him from before or not.

The forbidden deserting is removed by greeting with Salaam. So if you meet him and greet him with Salaam, then you will have done what is required of you, and you will have avoided the prohibited desertion. But if you intentionally refrain from greeting him for more than three, then you are sinful.

Allah knows best.