Why don't you put a For Sale lander up and wait for them to contact you and save the 20%.
What broker would end talks with both sides as the deal is coming together? If he was that
close to closing the deal ask him for the name/email of the legal department he was negotiating with.
It's always best when you wait for the buyers to come to you when dealing with large corporates like T-Mo.
Good Luck to you!