Dear Priyesh Mamnani,
1. I don’t think the transaction is leviable to GST because it is not in the course or furtherance of business (Not applicable if your company is in the business of selling old cars). However, in your case, you have capitalized it. Then it might fall under Entry 4(a) to Schedule II. Entry reproduced hereinbelow:
4. Transfer of business assets (a) where goods forming part of the assets of a business are transferred or disposed of by or under the directions of the person carrying on the business so as no longer to form part of those assets, whether or not for a consideration, such transfer or disposal is a supply of goods by the person;
Thus, it might be a supply of goods.
2. You may use NN 8/2018 -Central Tax (Rate) dated 25.01.2018, valuation is on the margin of supply. Make sure your transaction value (i.e. INR 10) shouldn’t be so low that the department may come and challenge that on the grounds of open market value. If the department challenges the same, you can take the benefit of INR 15 already paid for arriving at the open market value [refer valuation rule 2 or Rule 28] though it is margin of supply for valuation, still department may issue the notice w.r.t. OMV.
3. Going 6 years back, you cannot pay GST on INR 15.