Danielle from the Shopify Guru Team here.
The only way for a gift card's balance to be adjusted is for it to be used online in a Shopify store or through the Shopify POS. If you have an iPad or iPhone, the Shopify POS app is available to every Shopify merchant as part of their monthly subscription.
An alternative option is to disable the gift card and manually issue a new one at the lower balance.
Let me know if you have any other questions!
I certainly understand not wanting to switch the entire POS system over, but you could also use the app for one off purchases such as the scenario you described above.
Gift cards are treated as a method of payment - which is why only the last 4 characters are shown in your Admin. Removing funds from a gift card in your scenario may be totally legit, but could have serious implications on other stores.