ESPP Offset

Does anyone know what this is?  I've sold stock that my employer purchased post tax on my behalf over the years and all of a sudden I received a W2 and was told by my former employer that this was "offset".  The W2 makes it look like I received income and I have not worked for this employer for nearly two years.  Yes, I sold some stock and expect to receive a 1099-B for any capital gains/losses.  Any help is greatly appreciated.

 

Thank you