I too am coming from where Baldric is asking. I can understand how you weren't sure 100% how much you would get that month - doing extra hours etc., but surely you had a rough idea? I too can't understand how you can say you would be destitute if you had to pay it back in a lump - surely your monthly outgoings are the same and you afford it from whatever your "normal" salary is. However - the answer I would give is yes, they can take it back in one lump, but as others have said, it would be much better if they negotiated a reasonably repayment plan with you, particularly since you have explained to them that you were on leave, had worked extra hours, and thus werem't sure how much your nett income would be last month. Hopefully they will be accommodating to you.