If the Letter of Credit in this case referred to the document as issued by a bank then it is a committed payment where upon after the seller fulfilled all conditions/ clauses that required of him (the seller) the buyer's bank ( the bank that issued the Letter of Credit) will have to transfer the sum of money stated in the Letter of Credit (L/C) to the account of the seller.
The buyer will have a credit term of xx days (as granted by the bank) after the transfer of money to the seller, to pay back to the bank, with interest added.
So if an L/C, especially a "Irrecoverable Letter of Credit" has been issued, the deal is a s good as done, unless the seller cannot meet the conditions stated in the L/C.
A buyer can default payment to the bank, but the bank cannot withhold payment to the seller.