Valve has just added digital Steam Gift Cards 20 USD to Steam so he can send money directly to a friend’s Steam wallet. You can also use the same trick that you used to add the Steam Wallet amounts to send almost any amount, not just the default values.
Make sure your friend has a Steam account
If you want to buy someone a game who does not have a Steam account, they’ll have to set up a new account and download the Steam client.
Once they are set up, you should go to the Friends drop-down menu, select Add a Friend, search for their username and invite them to become your friend. Once they accept your request, you can send them a game.
Find the game you want to buy and add it to your cart
Locate the game you want to give in the Steam store, either directly in the client or through the web portal, and select Add to Cart. If you already own the game in question on your own account, there will be a note just above the buy option that says “Buy this game as a gift for a friend.”
Choosing a Gift Card
You can pick a gift card from the the Select Gift Card page, also accessible from Steam’s front page. In the US, your gift card options are $5, $10, $25, $50, or $100. If your friend is in a different country or uses a different currency than you, the amount you select will be converted to their local currency.
The background of each card is made of banners for the top selling titles in its price range. The image is dynamic, and updates with sales and new releases. Keep in mind that games may cost different amounts from region to region, and that this page doesn’t account for taxes, which are applied in some states and countries.
Choose your recipient
Select the recipient from your list of friends. You can choose to schedule the delivery at a later time and date if you want to set it up to arrive on a delay and be a surprise for a particular occasion.
Fill out the customized message
Personalize your gift by filling in your friend’s name and writing a message. This can be anything from a simple happy birthday or happy holidays to a heartfelt letter or funny message.
Complete the purchase
After personalizing your gift with a note, press Continue and you will proceed to the regular purchase screen, where you can proceed as you would to buy a game for yourself. Then, it's on your friend to accept the gift. Once they add it to their library, you’ll receive a notification.
Alternatively, you can try selling it as long as you have not used it yet. Try a gaming marketplace like https://www.igvault.com/Steam-Items , register and put up your spare key for sale there.