Another doctorate of economics here!!! All payments are included and passed first in the budget. It is not directly from the export earnings. It is paid from the internal revenues earned by the finance ministry.Foreign debt is paid back by export earnings, not from domestic revenue. If exports fall, even if domestic revenue remains constant, you can't pay back dollars using taka.
The incoming dollars are owned by the Bangladesh Bank, remittance or export earnings. BB pays the owners of those dollars in Taka, and the GoB has to buy those dollars by paying Taka to BB.
Now, if revenues are in short supply, the govt has to print new paper money and if it buys dollars with this new money, there will be equivalent inflation because this money will circulate through economic activities.
Please learn more and teach me. But, you are completely out of focus in your previous statement.