How can i put an UNUSED balance back to the gift card?

Since you specifically state you wish to return unused balance to a gift card I'll post this section from Check your Apple Account balance - Apple Support

If you have Apple Account balance that you can't use

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple Account balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support to request a refund. When you contact Apple, include the following information:

• Your Apple ID.
• The refund amount.
• Your current shipping address.

i need to put as an UNUSED balance back to the gift card,

That is not possible.

i have balance of 3k rupees in my itunes account.

Contact Apple Support directly, they will help you clear your Apple Account balance, click -> Get Support

Fill in the field >Tell us what's going on<

If you then want to change Country or Region, you must comply with all regulations described here -> Change your Apple ID country or region -> Change your Apple ID country or region – Apple Support (UK)

Hello,

I see you have remaining balance that you wish to remove from your Apple Account. Apple provides multiple ways for you to do this.

Read this support article for information about requirements for changing your Apple ID country or region --> Change your Apple ID country or region - Apple Support . In this article (and from Check your Apple Account balance - Apple Support referred to in the article) Apple outlines ways to remove account balance:

+ "Spend any balance remaining on your Apple ID"

If you then find your store credit is less than the cost of a single item and you still have a valid payment method such as a credit card for your old country, you can use the payment method to make up the difference in the cost of an item ( How apps, content, and subscriptions from Apple are billed - Apple Support ). Perhaps buy something such as a music track which you can download and also keep a backup copy. Remember that anything purchased may not be available for re-download in another country but as long as you have your own copy you will always have it.

+ "If you still can't change your country or region, or you have less store credit than the cost of a single item, contact Apple Support" using this link --> https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0998&symptom_id=20198

Apple removes your balance and keeps it.

+ "Check your Apple Account balance" refers you to Check your Apple Account balance - Apple Support

Note the sections "If you have an Apple Account balance that you can't use", and "If you plan to delete your Apple ID or change your country or region"