It is completely illegal to do this unless you had some employment agreement with them which gave the right to do this in this situation. But unless you had agreed to let the employer do this, they have no right to retroactively reduce or to withhold wages. For work you have done, you have to be paid per the terms of your employment at the time you did the work; if the employer won't pay, you could sue them.If they want you, they are free to make it worth your while in some way--for example, to offer you bonus or some other sum of money to keep working or servicing clients for three weeks. But that is the proper way to do this--not to threaten to withhold wages you have already earned.