Difference between Dain and Qardh, Delayed payment, Immediate payment


1. What is the difference between Dain and Qardh? 
2. If a person purchases a pen from a store and walks away with the intention of paying a little later, will this fall under the category of Dain or Qardh? In this case, no time has been fixed for payment of the goods. Is this transaction in order? If a purchaser buys the goods, but no arrangements have been made to pay in "terms", will this be considered a cash transaction, except that the buyer is delaying payment? 
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