No one has really covered this point so far...why exactly did the Cutoff get a penalty? From the information provided the Hero bet 6k, Cutoff made a mistake and thought it was 1.5k and threw in that amount. I can't see why that would be grounds for any kind of penalty.

On the main issue (and based on the information about the situation provided so far) I would say the Cutoff should be made to call the full 6k. In the OP I read that the Cutoff said the word "Call" on the River but from later discussion it seems that he didn't actually say it and just threw in the 1.5k. I don't think that matters and it should still be a call of the full 6k.

The exact details of any ruling situation are important as too often things are missed out when stories are retold. Will be good to hear the TD's thoughts later on.