Update Contract Quantity VBAP-ZMENG with user exit MV45AFZZ

Hi Team,

We are looking for a solution for updating Contract quantity(VA42) every time a sales order created and invoicing done for the same.

for this i am trying to write a code in -

FORM userexit_save_document.*CALL FUNCTION 'ZOTC_CONTRACT_QUANT_CHANGE'" IN UPDATE TASK

EXPORTING
ex_bstnk = xvbak-bstnk
*CHANGING
xvbap = xvbap[]
.
iF sy-subrc = 0.
COMMIT WORK.
ENDIF.

i have written logic in Normal Function module to update xvbap-zmeng field for the Contract quanity - sales order quantity.

But unfortunately its not updating table VBAP-ZMENG field.

Please help, as its very urgent.

thanks

Surya