@Contraception,
Your fiancee is staying at your house, working to repay you 6000 Euro?
Is that correct?
I think every other woman would have stiffed you for the money and left,
for her to stick around and repay you shows that she's sincere.
None of us know what happened with her and her former boyfriend, but
if you feel that you cannot trust her ever again, then there is no point to it.