DN/CN should get auto adjusted with Invoice upon generation and saving, otherwise again user has to go to DN/CN window to adjust also there are chances that the user misses the adjustment and releases the payment for full bill value. In case any changes will be required then adjustment will be manually removed.
While creating any AP voucher, if any purchase debit / credit note is pending to be adjusted against the invoice, the system prompts the user for the same. This was released in 11.139.2. You can adjust there itself.
We are marking this feature already exist.