Not really interchangeable. As Peachey said, 'fulfilled' can have a different meaning, depending on context. You can use satisfied and fulfilled to mean the same thing when talking about a fully completed action or desire, something met fully and without reservation, but it sounds a bit pompous and forced, e.g:
'Has your order been fulfilled?'
'Yes, my order has been fully satisfied'
I'd avoid that kind of usage.
In your example, satisfied is the only fitting word of the two. Here, fulfilled would have the sense that his paying you back 'fulfilled' you in some complete fashion, met a lifelong goal that had real significance to you in some profound way.