Gordon-Lime-Penguin Hi Gordon,
merchants can only send you money to your MasterCard, if there was a payment linked before.
So if you pay something for 20€, but an article for 10€ is not available, the merchant can refund you 10€ or the hole amount.
But a normal transfer to the MasterCard is not possible, this is not possible with probably any Debit MasterCard.
So you will need to use your IBAN for that.