Amount for a verbal raise in NL

[ThinkQuick]

I believe that this may have been discussed long ago, and there probably would have been a disagreement, but I can't find any posts about it.
This has come up a few times recently for me in live play and not always settled the same. I'm quite curious about what the actual ruling should be.

Player A makes a bet, lets say $55.
Player B says "$100".
It's that simple - what is the ruling on Player B's 'raise'?