let's say there are $500 in the pot before going to the river

Unless A was AI on a previous street, he would have a right to the whole pot.

A 350 - let's say A is SB for 25.
B 650 - B is in the BB, 50

pre flop A calls B checks - pot = 100

flop A bets 200, B calls - pot = 500

A now has 100 left, if he goes AI and is called the pot would be 700 and A would win the whole thing if his hand is best.