I'm amazed by the number of posts from people being overpaid on Salary/ Revenue refunds etc. that think for some un-imaginable reason they are entitled to hold onto the funds from the error. As my name suggests I wouldn't be flush with money but by god my name is worth alot more, I certainly wouldn't consider holding onto funds like this which were down to human error, why is it so hard for people to give back what is not theirs without trying to find ways around it. Think about if you were in a shop and gave in €50 and got change back of €10 and then was told by the Shop Keeper to come back each day for the next forty days and they'l give you a Euro a day, wouldn't be acceptable would it so why do you think your Employeer should have to wait for what is theirs. Also you'l more than likely be depending on this Company for a reference, what kind of reference would you think you'd get if you make this hard for them?
It's not your money, you haven't spent it, so just give it back!!