Plus when they resigned him recently, they originally didn't sign his tender. He then told his agent if the Bills still wanted him, to make a deal happen. He wants to be a Bill. His contract will not be expensive, the actual savings when you really break down the cap #s would probably be less then a million (not a cap expert so I could be wrong).
Paying a little extra for a known, proven commodity that wants to be in Buffalo is huge. When his # is called, he makes plays.