One way ..

In Sheet1,
In C2:
=IF(ISNA(MATCH(A2,Sheet2!A:A,0)),"Not
found",B2-INDEX(Sheet2!B:B,MATCH(A2,Sheet2!A:A,0)))
Copy down. Adapt to suit.

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Max
Singapore
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xdemechanik
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"CP" wrote:
I have a project where I need to compare month to month data from two sheets
and then make further calulations

Sheet1 = current month
ColA = Client
Colb = Value

Sheet2 = previous month
ColA = Client
Colb = Value

I require sheet1 ColC to show difference in value from previous month
(sheet2) to current month (sheet1) using the value in ColB but also matching
the customer.
If no customer match show a text value "text"