Like Zacmaster, I'm a bit confused by your question (or lack of one). The body of your post seems unrelated to the heading.
If you're asking whether your employer has the right to demand that you repay money which was overpaid to you as salary,the simple answer is 'Yes'. If you simply hang on to it, with the attitude of 'Tough, I'm keeping it', it's possible that you could be committing a criminal offence (which could lead to a fine or imprisonment). However, if you acknowledge the debt and simply state that you can't repay it all at once, it will be a civil dispute and the employer can't do anything except to pursue you through the civil courts. (If you really can't pay the money back at the moment, that wouldn't help your employer because civil courts try to avoid making orders which are impossible to carry out).
If I've mis-interpreted your post, or you need further advice, please post again with some clarification as to what your problem is.
Chris