I can't tell you where the 'absolute' line is on selling assets but the one thing that has always been enforced is that cash cannot be the largest component of the deal. That's a slightly arbitrary judgement that the Director of Player Personnel makes but in short you cannot outright sell a player or do something like trade cash and a 9th round pick.
As a few GM's will testify such trades have been rejected in the past.
As for the absolute limit of cash in a deal it's the same as that in the NHL ... I just can't remember how much that is atm