When I was a marker, a student came to meet me to complain that I did wrong in the assignment marking but he didn't bring his assignment. I assumed that what he claimed was true and apologized. Then he just kept complained about something and then asked if I accepted that my marking was bad, and I accepted. He claimed he would tell the university and they would do something to me, and then asked my name and contact information and left. Later I found that what he even talked to the wrong TA. Was this student misbehaving and should I have kicked him out of my office in the very first place? I was only told by the instructor to meet with students to talk about marking issues and have no other idea.
If this happens in the future, you should refuse to discuss the marking without the assignment in front of you. There's no need to be confrontational or assume anything suspicious is going on, you can simply politely say something like:
I would be happy to discuss this, but we can't have an informed discussion without looking at the assignment. Please come back later and bring it with you.
It's not clear from the information given whether the student was trying to get away with something, or was just disorganized, mistaken, etc.--nor does it matter at this point. Whether his intentions were good or bad, it doesn't make sense to discuss this at all without looking at the work.
Also, don't let an aggressive student push you into admitting something or making changes. If you see a clear mistake, then it is good to admit it. But beyond this, you need to be careful not to be pushed into doing things.
If you find it difficult to deal with a confrontational student, a good strategy is to defer the decision. Something like:
I understand the point you are making, but I can't make a decision right now. I need to go back and [review the other papers/discuss with the other TAs/discuss with the instructor/etc.].
This will enable you to put an end to endless objections by the student, and then make a rational decision away from the pressure.
If the situation continues to escalate, you may need to tell a student that abusive behavior is not tolerated and you can't continue the discussion.