When the ships are lost at sea, Shylock insists that he must have a pound of flesh as the contract demanded. —Antonio is spared only because of technicality: the contract did not say Shylock was entitled to any of Antonio’s blood. Thus, he cannot take a pound of Antonio’s flesh unless he can do so bloodlessly -- an impossibility.
This phrase is used to describe someone’s insistence on being repaid, even if the repayment will destroy or harm the debtor. Example: Sure, that initial low rate for a credit card is tempting, but the credit card company will want their pound of flesh when you get over your head in debt.