To manage returns of your products made to the consignment store, you can raise a credit note directly from the invoice of that sold unit, and identify the quantities to return to your consignment store's inventory.
Here you can enter in the quantities to send back to your CONSIGNMENT stock as this stock has been returned to the consignment store and held by them.
This will not create an adjustment within your product stock inventory as with when you send units back to stock within a return authorisation ; as this stock will remain within your consignment customers possession.