Hi there Hara!
Don here from Shopify!
If you're referring to sales made online rather than exchanging an item sold via POS then it's not currently possible to exchange for credit as you suspected.
For orders made online that you wish to return to credit instead of refunding to the payment method used you can check out the FreshCredit app.
This app looks like it should allow you to refund orders made online to in-store credit rather than returning the funds to the card or other payment method used. :)
As with any app in our app store, you can reach out to the developers directly using the email address or contact form listed on the app store page for any support questions you might have on their app.
All the best!