This is all I found, from Peter King's column.
"Say they accept Favre back, then try to trade him. They're going to get, what, a third-round pick from Tampa Bay or the New York Jets or Washington for him? They'll forever be known as the men who sent the greatest Packer packing. And there's no guarantee Favre would agree to go to just any team if he chooses to come back. Don't put it past him and agent Bus Cook to refuse to report to a team Favre doesn't want to play for."
Nightmare in Green Bay (cont.) - Peter King - SI.com