Is it illegal for an employer to do this?....?

I am a salaried manager i am required to work 50 hrs min. I usually work 50 to 55. A couple weeks ago i worked 48 hours. When i checked my balance in my bank acct they deducted a full days worth of pay. To me this seems wrong. I checked out a website and it said they cant deduct pay for a partial day just wondering if im understanding this right. Any thoughts on the legality of this and/or suggestions of what should be done?