Boss also will get $2 million that was guaranteed by the Raiders for 2012 before they cut him earlier this week.
Boss signed a four-year, $16 million contract last summer to replace Pro Bowl tight end Zach Miller, but wasn't utilized much in former coach Hue Jackson's offense.
In 14 games, Boss had 28 catches for 368 yards and three touchdowns, all the fewest of his career since his rookie season.
The Chiefs were caught flat-footed last summer when tight end Tony Moeaki tore his ACL during training camp.
The 6-foot-6, 255-pound Boss was a fifth-round pick by the Giants in 2007.
Information from The Associated Press was used in this report.