I had a festive ambassador for 0.88. Someone bought it for a reclaimed metal, and 0.55 credit. Why can't I withdraw this 0.55 credit?

Because the site only lets you withdraw credits in units of 1 ref (I assume that's because scrap and rec would take a lot more space to store)

If I have space and spare metal, then I usually keep some trades in my shop where people can trade ATM credits 1:1 for scrap or rec. Take a look.