I believe this would be the applicable rule from
Robert's Rules of Poker:
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7. Because the amount of a wager at big-bet poker has such a wide range, a player who has taken action based on a gross misunderstanding of the amount wagered needs some protection. A bettor should not show down a hand until the amount put into the pot for a call seems reasonably correct, or it is obvious that the caller understands the amount wagered. The decision-maker is allowed considerable discretion in ruling on this type of situation. A possible rule-of-thumb is to disallow any claim of not understanding the amount wagered if the caller has put eighty percent or more of that amount into the pot.
Example: On the end, a player puts a $500 chip into the pot and says softly, “Four hundred.” The opponent puts a $100 chip into the pot and says, “Call.” The bettor immediately shows the hand. The dealer says, “He bet four hundred.” The caller says, “Oh, I thought he bet a hundred.” In this case, the recommended ruling normally is that the bettor had an obligation to not show the hand when the amount put into the pot was obviously short, and the “call” can be retracted. Note that the character of each player can be a factor. (Unfortunately, situations can arise at big-bet poker that are not so clear-cut as this.)
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