Read this post below. They already have stacks of blank absentee ballots ready to go. They just don’t use them unless they get a call.
Once the ballot is separated from the envelope, who would ever know when or if it were stamped?
http://www.freerepublic.com/focus/news/3498767/posts?page=94#94
That would be done before the election. I used to live in Washtenaw County - Democrat Central. I was a poll challenger there. At the end of poll day, every vote there has to match the pollbook and is sealed. The same goes for AVCB (Absentee Voter Counting Board).
I can't speak for other states, but fraud in Michigan - if it occurred - would have been done already.