http://www.cbafaq.com/salarycap.htm#Q28
28. When a player signs mid-year for the rest of the season, is his salary pro-rated? What is a "Rest-of-Season" contract?
A "Rest-of-Season" contract is exactly what its name implies -- it is a contract signed after the start of a season, which is in effect for the remainder of that season.
With the exception of minimum-salary contracts, salaries do not pro-rate during the season. However, the exceptions that enable players to be signed are subject to pro-ration (see question number
26). For example, a team that has not used its Non-Taxpayer Mid-Level exception could use its full amount to sign a player on the last day of the regular season. By that day this exception will have pro-rated down to less than half its original value. However a player could sign a rest-of-season contract on that day for the entire pro-rated amount, earning it all for just one game.
The minimum salary begins to pro-rate on the first day of the regular season. The minimum salary on a Rest-of-Season contract is based on the fraction of the season remaining when the contract is signed. For example, if there are 177 days in the season, then a minimum salary contract signed on the 60th day of the season is worth 117/177 of the full minimum salary amount.1 If a player signs a multi-year minimum salary contract partway through a season, then the first season is pro-rated and the salary in subsequent seasons is the full minimum salary.
Raises are based on the actual salary in the first season of the contract. For example, if a free agent signs a two-year contract on the last day of the regular season and the salary for the first season is $1 million, then the second season salary may range from $950,000 to $1.05 million (plus or minus 5% of the first season salary).
10-day contracts (see question number
80) are also pro-rated. The salary on a 10-day contract is based on the number of days actually covered by the contract (a 10-day contract lasts 10 days or three games, whichever is longer).
The D-League salary in a Two-Way contract (see question number
82) is pro-rated as well, if the contract is signed during the season. In addition, the 45 day maximum for NBA service on a Two-Way contract is also pro-rated.
1 It is not considered a minimum-salary contract if a player signs for more than the pro-rated minimum -- i.e., the player cannot be signed using the Minimum Player Salary exception (see question number
25), the player cannot be traded as a minimum-salary player (see question number
88), and the team cannot receive reimbursement for a portion of the player's salary (see question number
22).