COMPARE Value differences between 2 columns by DATE

You can start by creating measures to calculate the amounts in the periods you want to compare. The years in this example could be fetched from selections in slicers, current year or something else in the below formulas.

QuantityCurrentYear =
VAR CurrentYear = 2019
RETURN
    CALCULATE (
        SUM ( Orders[Quantity] );
        KEEPFILTERS ( Orders[Year] = CurrentYear )
    )

QuantityPreviousYear =
VAR PrevYear = 2018
RETURN
    CALCULATE (
        SUM ( Orders[Quantity] );
        KEEPFILTERS ( Orders[Year] = PrevYear )
    )

Then create the measure that groups on vendor and part and calculates the difference:

QuantityDifference =
SUMX (
    SUMMARIZE ( Orders; 'Orders'[Vendor]; Orders[Part] );
    [QuantityCurrentYear] - [QuantityPreviousYear]
)

(if you use english settings, replace ; with ,)

Best Regards // Ulf