for a minute/just a minute

  1. Step in/into my office for a minute.
  2. Step in/into my office just a minute.
    Please correct the above sentences.
  1. Step into my office for a minute.
  2. Step into my office for just a minute.
  3. Step into my office, just for a minute.

Beeesneees,
“Step into my office just a minute.”
Is this sentence wrong?
“Just a minute, I am coming.”
Is it correct?
“For a minute, I am coming.”
Is this also correct?

‘Step into my office just a minute’ would, at the very least, need a comma after ‘office’. However, the meaning would be ‘just for a minute’ so it is better to include it.
‘Just a minute’ is most often an equivalent of ‘wait one minute’ so your original sentence sounds as if you are saying ‘step into my office, but first wait a minute before you do so.’

‘Just a minute, I am coming’ means ‘wait a minute, I am coming’ (I am coming and will be with you in a minute).

‘For a minute, I am coming’ is incorrect.