Inconsistency in delivery qnty and billing qnty

Hai Gurus,

I have scenario of transfer of goods from plant to Depo

There is a inconsistency in quantity between the outbound delivery and proforma invoice.

For ex: outbound delivery has been done for 2028 nos and while doing Proforma invoice system took 1932 nos.

As a process after outbound delivery proforma invoice is been generated.And goods move out.

In proforma invoice there is no way to change the quantity and this inconsistency came as all the fields in proforma invoice is graded.

Then also this inconsistency came.

Please suggest me resolution / sap notes so that it could be tackled

With Regards