nco garnishment

I qualified for hardship with NCO in the fall. When I came up for review at the end of Feb. I had the same income which is $580 two times per month bring home. I received a letter stating that I no longer qualified for hardship because I made 4300 per month. I called to inform them of the error because they are going to garnish $378/mo from my check unless I voluntarily repay. I have 3 children at home to care for. I made a request for reconsideration, but I have now been told that unless I pay the $378 they will begin the garnishment even before they have the reconsideration hearing. If I have $378 extra dollars I would have no trouble paying my bills. I offered to set up an arrangement that I could afford without success. They were very rude and disrespectful. The calculation was their error yet I am being garnished for it anyway. The letter I got stated that if I wanted to repay to notify their office to set up payment arrangements; however, when I did that they would not make an arrangement less than the $378. It also stated I would have to sign and return a repayment letter within 10 business days. I spoke with them on 4/8, as of 4/14 the repayment letter had still not been sent to me. I explained that I would not get paid again until 4/30, but they said they would not wait that long. How can they begin garnishment when I have not been given the opportunity to return a repayment letter? Is it legal to hold that much money when my income was figured incorrectly? Is it legal that they are waiting to send the repayment letter until my opportunity has expired? Someone please HELP!!!!!