Extract data per ID form different table

ZanneMari 

You can create a calculated column in the Open PO table to check if the PO line item exists in the History PO table.

HasHistory = IF(
COUNTROWS(
FILTER(
HistoryPO,
HistoryPO[PO Number] = OpenPO[PO Number] &&
HistoryPO[Item ID] = OpenPO[Item ID]
)
) > 0,
TRUE,
FALSE
)

Create a calculated column to determine the latest state for each PO line item.

LatestState =
VAR CurrentStates =
FILTER(
HistoryPO,
HistoryPO[PO Number] = OpenPO[PO Number] &&
HistoryPO[Item ID] = OpenPO[Item ID]
)
VAR MaxState =
MAXX(CurrentStates, SWITCH(HistoryPO[State], "E", 1, "D", 2, "Q", 3))
RETURN
SWITCH(MaxState, 1, "E", 2, "D", 3, "Q", BLANK())

Create a measure to sum the values for the latest state without duplicates.

dax
SumLatestStateValue =
VAR LatestState = [LatestState]
VAR UniqueValues =
SUMX(
FILTER(
DISTINCT(
SELECTCOLUMNS(
HistoryPO,
"PO Number", HistoryPO[PO Number],
"Item ID", HistoryPO[Item ID],
"State", HistoryPO[State],
"Value", HistoryPO[Value]
)
),
[PO Number] = OpenPO[PO Number] &&
[Item ID] = OpenPO[Item ID] &&
[State] = LatestState
),
[Value]
)
RETURN
IF([HasHistory], UniqueValues, BLANK())

Finally, create a calculated column to calculate the spend left.

SpendLeft = OpenPO[Net Value] - [SumLatestStateValue]