I'm pretty amazed that after an apparent agreement someone would decide that you and him both getting 20% wasn't enough and he needed 24% at your expense.

That's a pretty ridiculous move for someone to make, in my view. It goes well beyond dollars and cents, I'd think. It's obviously not about the percentage points. Losing 4% isn't that big of a deal, and yet neither is gaining 4%. But asking someone who is roughly equal to you to accept this type of agreement isn't trivial. You should probably ask yourself why this person was willing to go through all this potential heartache including the personal issues such a move would cause for a measly 4%.