You need to prove that the funds you submit for POF are not encumbered by a loan and are not borrowed and in your case you would need to prove that you sold something to get that cash/money.
Sell your gold to a licensed and government gold reseller such as banks and NBFC's. Upon buying gold, the financial corporation will issue you receipts, certifying that you sold them the gold at a certain rate. Use that receipt and income as your POF.
