Of course they can redo the deal. They can restructure it so that he isn't getting it as much against the cap this year and guarantee more in the future. I may have misspoke about the guarantee money, but word was the Pats are frontloading the contract to make it impossible for the Dolphins to match. If they restructure it after a trade, then they have more cap room to do other things.
Again, the Dolphins are guaranteed a second round pick if they lose Welker. Why would they trade him for less to a division opponent?
If you won the lottery and you were going to get a million dollars after taxes in two months, would sell the rights to someone else for $500k to get the money now?