BKPF-XBLNR (Reference Field is not changeable)

Hello All,

We person a existing config, to allows alteration of field BKPF-XBLNR.

When I participate the papers manually F-02, I americium capable to alteration the BKPF-XBLNR aft the papers is posted

We person a case, wherever accounting doc is posted via Purchase Order and owed to immoderate crushed we privation to alteration the BKPF-XBLNR field, and strategy is not allowing to change. Is this happening due to the fact that the accounting doc is posted via Purchase Order? If we privation to alteration these kinds of papers is determination immoderate different way?