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",