@Mihal10 Did you try editing the Payment Method settings? I'm not entirely sure if it's possible with this particular kind of travel credit, but that's the first place I'd check.
If it doesn't work, and your upcoming trip is with another person, you could organize the booking through their account (all the better if they don't have an account and you can get the referral bonus). But what I would do in your situation is use the travel credit now to make sure there's no risk of it expiring, and then transfer the appropriate amount to the group on whose behalf you placed the original booking.