You have to use a valid card in the current region of the device to pay for the unpaid purchase. If it has previously failed to process the charge it is likely not considered a valid payment method anymore, so it will require a different one. You will not be able to pay for an unpaid purchase in one region with a card of a different region.
Normally I would recommend using a Gift Card purchased in the original region to pay for an unpaid purchase. You won't be able to change regions though if there is a current balance on your Apple Account, which would likely be the case after the unpaid purchase was paid, so you would need to contact Apple directly to clear that balance left over if you are unable to spend it.
Contact Apple Support - Apple Support