id like to inform you of the Dodd-Frank act enacted by congress (effective July 21, 2010). you can search it on google and either read the summery or the entire bill in long form. the point your looking for is the requirements regarding credit cards and debit cards to protect consumers.

it is unlawful for a business to charge over a $10 minimum on credit cards. and it is unlawful for a business to charge ANY minimum on a debit card. business can only cover credit card service charges by offering a cash discount to consumers. refusing to accept payment unless more items are purchased is extortion.(ie: already ate your food...)

however Visa, Master card, and american express's merchant practices forbid a merchant from placing minimums on credit purchases. violation is a breach of contract.

if you come across this contact the bank your card was issued from, appropriate legal action will ensue. if a merchant is refusing to accept payment on items/services already rendered/used call the state police or your attorney, as a consumer you do not have to accept this practice.