If he puts out either amount in chips then it's a moot point.
If he says "Raise. 2000." I'd be inclined to believe his intent is to make it 2000 straight.
If he says "Raise 2000." Then I'd assume he means to raise it 2000 up to 2400.
In either case I'd ask him to clarify for the bet at hand and to try to be less ambiguous in the future.
In general if I have to make a decision one way or the other I'm going to err on the side that is to the player's disadvantage, in this case making him put in 2400 total. By giving the player worst case scenario it does not allow that player to benefit from a possibly intentionally unclear wager hoping to angle shoot.
Yep. If there is a pause, then it is a bet of 2000. With no pause it is a bet of 2400.