If you are worried about 'due to be paid'/ 'due' and 'pending' all are good and used appropriately.
He has six days due to be paid- is fine.
He has six days pending / due in his last month- are fine.
However out of context -He has six days due to be paid in his last vaction. Not sure what you mean with the 'in his last vacation'. Is he being paid for the vacation, or is it to be paid within the time of his vacation. This isn't clear, but mostly due to lack of context.
Pending, due, and due to be paid are all ok.
All the best
Student or Learner