It depends on whether you create a Closed or Open Fixed Virtual Account, the difference will be as following:

- Closed FVA
There will be an expected amount and VA can only be paid exactly the same as set amount before.

- Open FVA
VA can be paid by any amount. Note: the minimum and maximum amount set by Bank will still be applied.

Notes:

BJB and BSI only support closed amount, thus you need to specify the amount when create the FVA
If you do not specify the parameter "expected_amount" the system will throw error

"error_code":"OPEN_VA_NOT_SUPPORTED_ERROR",