How to update actual fulfillment quantity in sales order by integration with OTM

Summary:

I have OM integrated with OTM and I am adding step in fulfillment process after shipment for customer acceptance

I created sales order with 10 qty and I shipped 10 from warehouse (ship confirm 10) now line status will be shipped

through OTM I integrated that customer only accepted 8

How to automatically update shipped quantity to 8 and backorder 2 to inventory

I did not want to create sales return with 2 as customer already accepted only 8 and he wants one invoice with 8

I was only able to do this with keeping shipment status as staged then in this case I was able to create backorder