http://pbsl.ijbl.net/check.htm
This should have the rules FBB uses built in, to wit:
1. No matter cap situation, a team may always decrease its salary in trades
2. A team increasing its salary must have either (a) enough cap space to take on ALL of an increase in salary or (b) must have sent out at least 80% of the salary it is receiving (or if you prefer, a team can receive up to 125% of outgoing salary)
Reminder: Salary cap space is not considered salary for purposes of 125% match. You cannot mix and match salary matching and cap space, it must be totally done with one or the other.
(Note: Trade rules thread as of this writing says salaries must be within 10%. If we are using FBB built in checks this is wrong - should be 25%)
DISCLAIMER: I make no guarantees this is perfect, but the rules are programmed more correctly than that other link, which does let you use cap space for salary matching.
-per WigNosy