When you have rounded your number values in Excel, you may want to add a unit indicator to the end of your new value.  To do this, simply add the text of the new value within quotation marks in the custom number formatting syntax.

0,000,, “M”;(0.000,, “M”)

Note, in this newly formatted cell, you can type whatever numeric value you want without typing “M” at the end of the entry.  It will still show up in the same format as 2.312 M, or (2.312 M) if negative.