Headline is slightly misleading. He doesn't have to pay back his base fee and and actual material costs. What he does have to pay back is the additional money he was supposed to spend on the work, plus legal fees. When he pitched the work and got the commission he said an annual average danish salary in cash would be integrated into the work, and the museum agreed to give him this money on top of his fees. When the money wasn't used in this way in the work the museum received, the museum decided they wanted that money back.