For example a letter of credit could be used in the delivery of goods or the completion of a service. The seller may request that the buyer obtain a letter of credit before the transaction occurs. The buyer would purchase this letter of credit from a bank and forward it to the seller's bank. bank guarantee, like a line of credit, guarantees a sum of money to a beneficiary. Unlike a line of credit, the sum is only paid if the opposing party does not fulfill the stipulated obligations under the contract.