So this effectly means no Cassell in the playoffs since even if the Kings waive Sam, he couldn't rejoin the Celtics for 30 days which would put it after the March 1st roster deadline for being playoff eligible.
I don`t think that is true. He just needs to be waived before March 1st. A player is still eligible for the playoffs if he signs with a team until the the last day of the regular season...
Correct.
I was pretty sure a player traded to another team that is then bought out or waived can not rejoin his former team for 30 days which would then exclude him from being added to a post season roster.
You're correct that Sam couldn't rejoin the team for 30 days.
You're incorrect, though, in thinking that that 30 day limitation precludes Sam from joining the playoff roster. Players only need to be bought out prior to March 1 -- rather than signing with a new team prior to March 1 -- to be eligible for the playoff roster. If Sam was bought out on, say, February 25, he couldn't join the Celtics until March 27 or so, but he'd still be able to play in the playoffs.
Here’s where things get interesting. If the team was planning on bringing Sam back, financially this may make sense. If Sam stayed on the team the entire season, the Celts would have owed him roughly $1.6 million in salary plus luxury tax. If he comes back with roughly 20 games left, (and if and only if the league treated Sam like any other minimum-salaried player) the team would only owe him approximately $389,000 in salary plus luxury tax: ($797,581 * 20/82s, doubled). Thus, if their plan is to only have Sam play a limited number of games prior to the playoffs, this trade would allow them to do so, while getting them approximately $205,000 in savings after paying Sacramento off ($1.6 million combined salary pre-trade – $505,666 paid salary to date – $389,000 current luxury tax – $500,000 to Sacramento = $205,334).
This assumes that the league treats Sam as any other minimum salaried player the team adds for the stretch run, rather than viewing this as a means of circumventing the salary cap.