You can use the residual voucher to pay for changes to your booking or for other tickets. But it seems they are only good for travel tickets. There are.some FAQs here:

https://www.qatarairways.com/en/help...#travelvoucher

You may also be able to get a refund but there may be fare penalties to pay depending on when you cancelled the original booking (eg. no show), and the 10% uplift will not be refunded. One or other of those coild wipe out any residual cash value depending on your original tickets. It probably doesn't hurt to contact them to ask what any refund amount might be, but be sure to say it is only an enquiry at this stage