Upvote:1
OTAs usually allow third party payments, but it is normally the 3rd party that does the booking ie: your relative makes a booking and pays for it with on your behalf. That way their address, contact and payment information is in the record and you are listed as the room occupant only.
In your case, you have already made the booking so your address and contact info is part of the booking. This would cause their payment to be declined due to address mis-match.
Why not check to see if you can make the same booking again (see if the rooms and rate are still available). If they are, cancel your current booking and then get your relative on the phone and have them make / pay for a new booking.
Otherwise your best bet is to call booking.com and see if they will allow a third party payment from outside the original booking. Some OTAs do have a payment form that can be filled out and signed, so that an outside party can pay.