Redundancy money paid by mistake?

The City Council I am working for issued me with a redundancy notice 3 months ago. In the end they were finally able to find me a new post but got my redundancy money paid. As I raised the issue with my line manager, she told me to keep this money in my back account until instructions were given to me from HR. One week ago a letter came through the post saying that I had to come into an agreement with my line manager as to how the money would be paid before 3 months time. Because of this letter I decided to expend part of the money and pay for it in installments over a period of time. However, a fax came through today at work saying I have to give this money back straight away or else I won't be included in the payroll for my next wages. Because this is not my mistake, and because the contradictory instructions given to me in these letters, I am a bit worried I might come up against something when I have never done anything wrong. There are other people like me in the same situation at work who have also been required to give this money back. All I am asking is where the Law stands in all of this and whether they can issue me with a compulsory dismissal because of this. My boss says if I don't give this money back to HR before the end of the month I will automatically sack myself. I don’t refuse to pay for this money, but I haven’t got the full amount of it.