Hi,
Check back posting allowed in t.code:MMRV for your company code,If yes then proceed.
Also check to refer table MARV in t.code:SE11 for your requirement.
NOTE:
For Negative posting ,find followings.
Check box selected for "negative stock permitted" in processing parameters segment in OBY6 t.code for your company code.
&
Check box selected for "negative stock" for your plant(valuation area) in OMJ1 t.code.
Regards,
Biju K