Who bears the loss of the merchant who paid for the goods to the customer's bank, and then the bank went bankrupt, which prevented the merchant from recovering his dues?

What you have asked about has three possibilities:

1. The customer may have deposited the amount into your bank account, in which case the customer's obligation is cleared, and the amount is now the bank's responsibility.

2. The customer may have referred you to the bank with the amount and you accepted the transfer, in which case the customer's obligation is cleared, and the amount is now the bank's responsibility.

3. The customer may have placed the amount in the bank for it to be delivered to you or to your account in another bank. In this case, the customer's obligation is not cleared unless the bank that received the amount was an agent for the seller or an agent of his agent in receiving the amount.