It is common for a letter of credit issued by Bangladesh to modify the amount, but it does not say that the amount cannot be modified, hehe.
You have two choices now:
First, don't cancel the letter of credit, and submit the documents according to the requirements of the letter of credit. But if the letter of credit requires the buyer to sign the acceptance certificate and other soft terms, you can't do this.
Negotiate with the buyer to open a new letter of credit, and then you can agree to cancel the original letter of credit.
The key to your case is whether there are soft terms in the letter of credit he opened at the beginning. If so, you are in a weak and passive position in the negotiation. If not, you are in an active position. As long as it is not due, you have a lot of room.
A few suggestions for your reference.