Salary matching is the rule that a basketball team over the salary cap can only take back trade salary within a set margin of what it sends out, so trades cannot be used to add payroll freely.
In HoopSim every team in a trade has to pass the rule for its own payroll tier, checked when the trade is proposed and again when it executes. Under the cap: your cap room or the over-the-cap allowance, whichever is more. Over the cap: sending up to $7.5M lets you take back 175% of it plus $100K; $7.5M to $29M, what you send plus $7.5M; more, 125% plus $250K. First apron: 110% plus $100K, and your biggest outgoing contract must match alone. Second apron: what you send plus $100K. Picks carry no salary, and no trade can finish above the hard cap.
Rulebook: Salary matching. Related: apron, salary dump, trade exception.