the disavantage card

went to buy 3 for 2 items but was told the amount came to more than what's on my card, I could use 2 of the items against my advantage card and pay for the third (even though it was buy 2 and get third free), but can't part pay as it was over the amount on my advantage card. was told you can only buy up to the amount on card but can't pay the difference if you go over the amount..WHY??