Hello Jim,
This is like so many other situations our players keep putting us in, that is: being unclear with their intentions. The way I see it, if the dealer doesn't freeze the action until the player announces his wager, or pushes it forward, be prepared for anything. What you described happens every day in every poker room, and on every betting street. Example: Player A bets 100, Player B says "raise 300."
is the bet now 300? or 400? The options to the players are: 1 push out the intended amount in one motion...as long as it's not a single oversize chip, of course.
#2 Say raise and push the total amount forward in one movement. #3 Say "raise 300 more"...or #4 Say "raise to 400."
TDA #37B addresses consequences to the players that react before the amount is clarified. You might also look to TDA #49 and rule it must be the lesser amount.
It's probably not the definitive answer you are looking for but, until the players let us know how much they want to bet or raise, they will have to suffer the consequences when we rule their wager is not what they intended it to be. This is one of the reasons why I'm always looking to the dealer to immediately react and get it right before substantial action occurs. i.e. Player A bets 100...Player B says "raise 300!" Dealer: "hold it!...I actually hold my hand up to Player C (like a traffic cop)...then I will address Player B and make sure we understand his bet before we proceed. It's not a rule, but it's the best way I know to prevent other players from re-raising or betting the wrong amount. I really want clarification before substantial action.
That's it...hope it helps a little.