Credit memo process

Dear Experts,

we made a transaction as follows in our concern.

PO made for 100 pcs of material @ Rs 100/pc = 10000

GR made for 100 pcs wrt PO

Invoice posted for PO = 10000

Now after inspection, 10 pcs are damaged in transport. we want to send back this 10 Pcs to vendor. How to post this transaction and which functionality?

Explain the functionality of Credit memo, subsequent credit and subsequent debit with the above example.

If transport cost is added as unplanned cost, which functionality to use