Odoo 19 Repairs has an under warranty option on the repair order for this case. Parts are still consumed from inventory, so stock stays correct, but the customer price is set to zero.

• Create the repair order from Repairs, select the customer and the product, and the serial number if you track one.
• Tick the Under Warranty option on the repair order before adding parts.
• Add the spare parts on the parts lines with the add type, so they are consumed from the repair location when the repair is done.
• Start and end the repair; the parts are moved out of stock and their cost remains visible in inventory valuation.
• For out-of-warranty repairs, leave the option unticked and use the button to create a quotation, then invoice it through Sales.

This way your warranty cost is measured through consumed parts, and paid repairs follow your normal invoicing and payment process.