Sir,
1. Under GAT Act, levy of tax is on supply. According to Section 7 (1) (a) of CGST Act, 2017 "the expression “supply” includes all forms of supply of goods or services or both such as sale, transfer, barter, exchange, licence, rental, lease or disposal made or agreed to be made for a consideration by a person in the course or furtherance of business." Since exchange of goods is also included in the definition of supply, supply of a new component or part in lieu of non-functional or damaged goods will be treated as exchange (free of cost) and the same is liable to gst.
2. You have to issue a regular taxable invoice for the said supply and have to pay the appropriate gst on such supply.
3. if the person who returns the goods as purchase return is a registered person under GST Act he has to return the goods under regular taxable invoice on payment of appropriate gst. According to Section 34 (3) of CGST Act, 2017 "where a tax invoice has been issued for supply of any goods or services or both and the taxable value or tax charged in that tax invoice is found to be less than the taxable value or tax payable in respect of such supply, the registered person, who has supplied such goods or services or both, shall issue to the recipient a debit note containing such particulars as may be prescribed under rule 53 of CGST Rules, 2017. Therefore purchase return cannot be done on debit note.
If the person who make purchase return is an individual or who is not procuring such supply in the course of or furtherance of business no procedure is available at present. Since the individual will be unregistered he cannot return the goods under the cover of taxable invoice.