Here is the exact language from the CBA, which can be found here:
CBA-Download - NFLPlayers.com
Section 9. Limitations on Salary Forfeitures:
(a) No forfeitures of signing bonuses shall be permitted, except that
players and Clubs may agree: (i) to proportionate forfeitures of a signing
bonus if a player voluntarily retires or willfully withholds his services from
one or more regular season games; and/or (ii) that if a player willfully takes
action that has the effect of substantially undermining his ability to fully
participate and contribute in either pre-season training camp or the regular
season (including by willfully withholding his services in either pre-season
training camp or during the regular season or willfully missing one or more
games), the player may forfeit the greater of: (a) 25% of the prorated portion
of his signing bonus for the applicable League Year for the first time
such conduct occurs after the beginning of training camp until the end of
the season for his Club, and the remaining 75% prorated portion of his
signing bonus for the applicable year for the second time such conduct occurs
during that period that year; or (b) the proportionate amount of his
signing bonus allocation for each week missed (1/17th for each regular season
week or game missed).