Hi Alex:
There was a thread on a somewhat similar situation not too long ago, perhaps someone can link to it.
At this time there is not a specific TDA rule that exactly addresses the situation, although parts of several rules may apply:
Rule 1 certainly applies, making a decision balancing fairness and the best interest of the game
Rule 3: player's responsibility to follow the action
Rule 37: Chips put in the pot in turn stay in the pot
Ultimately these are Rule 1 decisions. IMO there is great interest in the game in keeping the bets right. Further, the discrepancy is relatively small; traditional RRoP suggests that whenever a misunderstanding is within 80% of what the real bet should be, players might be held to the real bet. Not a hard and fast threshold, just a suggestion that RRoP makes. 80% of 3200 is 2560, very close to 2600 using that threshold.
Fortunately the amount here is small and I personally would hold both A and B to a total bet of 3200. They do have some responsibility to follow the action, and it's in the best interest of the game to keep bet amounts right. It's worth noting that in most of these type cases the underbets are relatively small, +/- one BB or so, which is a small price to enforce betting discipline.
Thanks for the interesting case. These specific situations will be discussed at Summit VII no doubt.