This is a little bit old but something similar happened not long ago in our poker club, and this was the rulling:
My rulling in a situation like this (even if a minimum raise is close to what an all-in means or is a big difference) is that Player B is forced to call the minimum raise of Player A and then decide to call or fold the remaining chips (if any) from Player A raise
Take this example:
Blinds 100/200, Player A stack 20.000, Player B 15.000
Pot is 2.000 until river,
Player B bets 500, Player A says raise (doesn’t get to say the amount), when Player B calls and flips his cards.
The next step in my opinion is to make Player B to put another 500 in the pot (mandatory), and return action to Player A to declare his raise and then Player B can decide what to do next (call or fold).
I don’t think is in the best interest and fairness of the game to force Player B to call an all-in from Player A, forcing this way to pot another 14.500 chips in the 2000 pot.