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You substitute some shellfish for collateral so your friend will lend you some money. Normally he would ask for something more valuable, but you’ve convinced him to substitute these shellfish for what he would normally take as collateral when lending to people.
You want your friend to lend you some money so you can buy yourself a car (か). You need money and a car a lot more than you need this pile of shellfish, so you’re happy to put them up as collateral for the loan.