Can Magento refund funds as credit to the customer? I need to refund the postage fee to the customer as a credit so he can use it for his next order, I can't find anything in the doc's about this.
It depends on your Magento version.
The enterprise edition allows you, when creating a credit memo, to 'Refund to Store Credit'.
If you use community, out of the box, it is not possible but can be managed through extension / development.
Have you managed to find a solution? If not, there is an extension that supports store credit as a means of refund https://www.magentocommerce.com/magento-connect/store-credit-by-amasty.html