Stock clear from 999 storage type after storage location de-assigned from WM

Client assigned one plant and storage location to WM and created one manual TO using movement type 712. Which generated negative stock in 999 storage type.

However later on client removed/de-assigned the same plant and storage location from WM. if i go to LI21 to clear off negative stock it creates material document but does not clear off negative stock. instead add stock in IM.

Kindly suggest how to clear this negative stock