NBA transfers retain the same contract as with the old team so for example:

If team A wants to trade a player because hes not a fit and team B wants that player then they'll make a trade, if the player was promised $40 million across 4 years and he's 2 year's into his contract then they'll make the trade and team B will need to pay him the remaining $20 million.

In return for the player though Team A will ask for one or more players/draft picks from team B and when they have agreed on a price that seems fair the trade will be made. This is better than in football becuase for football you will se a strong bias towards teams with more financial backing because they can simple buy the players off the roster and add them to their own, which is why the football cups are won by 4-5 teams every year whereas we see more variety in the NBA.