Billing Block field in Sales Order Header is greyed out

Dear Experts,

We have a Sales order with the combination of Delivery items and Service items, where service items are invoiced with reference to Sales order and Others are invoiced through Delivery. Sales order is fully invoiced and Billing block field at Sales order header is greyed out as per Standard behavior. When we cancel Invoice for any Service Item Billing block is enabling and whereas if we cancel for delivery Item Billing Block is still in greyed out and we cannot update the billing block at SO header Level. I'm an Abaper, so I'm not sure its a standard behavior or something to do to enable billing block after delivery item invoice is cancelled. Can you please help me here. When we debug program is checking the below condition

( VBUK-FKSTK = C' AND VBUK-FKSAK = 'C' ) OR
( VBUK-FKSTK = ' ' AND VBUK-FKSAK = 'C' ) OR
( VBUK-FKSTK = 'C' AND VBUK-FKSAK = ' ' ) AND
VBAK-FAKSK IS INITIAL.

Since delivery items invoice cancellation is not updating from 'C' to 'B' for field VBUK-FKSTK, where as it is updating

VBUK-FKSAK properly for service items.

Thanks,

Saleem.