SPOILER SPOILER SPOILER SPOILER ALERT
Don't have the book in front of me, but didn't the dying Baron give Tiffany a leather bag of old gold coins before he passed on? I always thought that was what she gave back to Roland. I thought she did this because true witches don't get paid with money, and giving it back was her way of ensuring that not only would her requests be kept but that Roland would always be in her debt, the same way the old Baron remained in 'moral' debt to Granny Aching.
Or maybe it was something else.