Unless laws have changed I believe Signing bonuses would be taxed in state of issue only.
Endorsement deals also are taxed on residency
Could you point me to the laws you are referencing? I was doing some reading on the jock tax and I don't think that would matter. The tax man would view the income as "earned" when the player played in games, and not necessarily on where they were paid.
Jock tax - Wikipedia
Brock Osweiler[edit]
A 2018
Sports Illustrated story on NFL players' tax situations began by examining the unusual 2017 tax situation of
Brock Osweiler. In
2016, Osweiler played for the
Houston Texans in the income tax-free state of Texas, giving him what writer Andrew Feldman called a "tax filing [that] was about as simple as you can get for an NFL player", paying other states only for the days he actually spent in those jurisdictions. At the time, he maintained his residency in Texas. During the
2017 offseason, he was traded to the
Cleveland Browns, but was cut after the team's last preseason game and picked up by the
Denver Broncos, with the Browns paying almost all of his $16 million contract for that season while he played in seven games for the Broncos. Feldman added, "So what did his tax bill come to? The experts say ...
they’re not sure. (emphasis in original)
[1]
In 2017, Osweiler was indisputably liable for about $150,000 in Ohio state income tax for the time he spent in the Browns' preseason training camp at Baldwin Wallace University in Berea. However, even though the Browns paid almost all of his season salary, an attorney with the Ohio Department of Taxation told SI that even though almost all of his income that year came from an Ohio source, the state could not tax him on income that was earned while he was not present in the state (assuming that Osweiler did not become an Ohio resident). According to several tax accountants consulted by SI, Osweiler was liable for $600,000 in Colorado income tax based on his time spent in that state, plus another $150,000 to the other states in which he played in 2017. One accountant suggested a different possibility:
[1]
One CPA suggests that Ohio and Colorado could both claim they deserve that $600,000, forcing an appeal and possible federal court case. The result could depend on whether the Browns paid Osweiler a lump sum before he left Ohio, or if he continued getting weekly payouts.(emphasis in original)
