Can you be "rearrested" after being released by "mistake"

My brother was found guilty of (multiple) DUI's and sentenced to 2 years and 2 months for the offenses. He was actually supposed to be sent to Prison, however spent the entire time in a county jail. He was released just a couple weeks shy of 2 years (due to 72 days work credit). His name was on the release notice board for an entire 2 weeks prior to being released, and when his day came, he was processed out and released. 2 days later a plain-clothed detective (from another county) came to the house and said "it was a mistake, you should not have been released" so he re-arrested him and took him back to the county jail. Is this even legal?