Quantifying Total Invoiced Per Month

ProfessorEgg28 , First create a new column

Incremental Invoiced Amount =
VAR CurrentInvoiced = [Invoiced Amount]
VAR PreviousInvoicedMax = CALCULATE(
MAX('Table'[Invoiced Amount]),
[Purchase Order] = EARLIER([Purchase Order]),
[Date Modified] < EARLIER([Date Modified])
)
RETURN
IF(
ISBLANK(PreviousInvoicedMax),
CurrentInvoiced,
CurrentInvoiced - PreviousInvoicedMax
)

Measure 1

Total Monthly Invoiced =
SUMX(
FILTER(
'Table',
MONTH('Table'[Date Modified]) = MONTH(TODAY()) && YEAR('Table'[Date Modified]) = YEAR(TODAY())
),
'Table'[Incremental Invoiced Amount]
)

Measure 2

Total Monthly Invoiced by Purchaser =
CALCULATE(
[Total Monthly Invoiced],
ALLEXCEPT('Table', 'Table'[Purchaser])
)

or

Total Monthly Invoiced by Purchaser =
CALCULATE(
[Total Monthly Invoiced],
filter(all('Table') , 'Table'[Purchaser] = max('Table'[Purchaser]) )
)