Article XIV, Section 8(b) of the Collective Bargaining Agreement states that
“[a] Club extending a Required Tender must, for so long as that Tender is extended, have a good faith intention to employ the player receiving the Tender at the Tender compensation level during the upcoming season.”
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In other words, according to the understanding of Bill Belichick, a team can’t apply the franchise tender to a player for purposes other than using that player on their team in the upcoming season, such as to facilitate a trade, to spark a later contract renegotiation, etc.
A team trying to trade the player contingent on a Tender being signed could provide the foundation for an argument that the team doesn’t have a good-faith intention to employ the player receiving the Tender.
Once the tender is signed (and thus no longer “extended”), the “good faith intention” requirement evaporates, and the player can be traded.
discussed in this archived Florio article:
BELICHICK: PATS HAVEN'T TALKED TO PANTHERS ABOUT PEPPERS