i will address this individially

Create a New Return Document:

• In the Inventory module, navigate to Operations > Transfers and create a return for the specific vendor.
• Select the Vendor and Return Type as "Return to Vendor."

Add Multiple Products:

• In the return document, manually add all the products you wish to return to the vendor by selecting them in the product lines.
• This allows you to handle returns for multiple products from previous receipts within one document.

Reference Original GRN Costs:

• Unfortunately, Odoo does not natively support selecting previous GRNs to automatically pull in historical costs for the returns. However, you can add a custom Reference Field in the form to note the original receipt (GRN) or batch. This requires enabling developer mode and customizing the form view.
• Another approach is to manually adjust the return prices if needed, based on the product cost details in the return lines.

Generate Credit Memos:

• After processing the return, go to Accounting to generate a credit memo associated with the return. This credit memo can then be used to apply the financial return values ​​for the vendor.