Actually the amount which we pay by our credit card to the merchant
establishment and the actual amount credited to the merchant
establishment is different.
Every member has to pay some percent transaction charges depending upon
his cr card receipts this percent can be less for a merchant if he is
getting much payment by cr card and vice versa.
May be that Merchant has decided some limit of percent of profit he is
going to make on the credit card transactions. because if you are making
a payment less than 500 than may be the profit margin is less and he
don't want to minimise it further or can't bear that.