Is it legal for a Maryland employer to deduct wages from a paycheck simply due to their own oversight?

My wife signed a contract in early 2012 to make outlining her wage/hr to include modified compensation. When she returned from maternity leave, she wanted to work part time. Her regional manager stated she couldn't receive modified comp as a part time employee. They agreed (verbal agreement) to cut back to $2/hr. No new documents were signed. The manager stated that she would contact HR to make the change. Apparantly no changes were ever made. Fast forward to present, she is told that she is still getting the old wage rate and they need to cut her back, AND they are going to deduct the $2 hour retroactively over the past 4 months from her paycheck (they may try to deduct even more based on their interpretation). Is this even remotely legal? My interpretation of MD law, specifically employer wage deductions doesn't seem to fit this scenario. Additionally, she has the old contract with wage rates in writing. This new wage change has no written/signed documentation.