How did you fix it 2 years ago?  There is something not being setup right with cost or there is something that needs recosted.

it was fixed without doing anything. The PO was received the next day without any negative inv value error. Different Qty of the current PO were received without any errors, i have checked Unit Cost which was never modified or changed. i guess it is a bug or something, cant be a functional error or Negative inventory value. I wonder...