Can we change(reduce) Account Assignment qty after a partial GR?

Hi there,

Are we able to change(reduce) the line item in account assignemtn in ME23N after a partial GR?

Example

PO no:123 has item 10 - 10qty
Account assignment has 10 lines items

We'll do a Partial GR - 1qty

By right, the PO still has 9qty left right?

Let's say we would like to change from 9qty to 5qty. How can we do it through IDOC? I'm always getting error msg:

"Sum of quantities >5< larger than total quantity"

Any advice? Thanks