Hi Jaya,
MR8M cancels the full amount and references the invoice (or credit memo)
you are cancelling. If you want to cancel a partial amount/quantity of an invoice
you can do it by posting a credit memo through MIRO as the posting logic
is the exact same as per note 46564. Also check out knowledge based
article 1609927 which shows the posting logic of credit memos.
Hope you find this helpful,
Adam.