VLOOKUP should do it.

Are the rows in each sheet unique? i.e. each part number only appears once in each sheet?

Lookup a part number from one sheet in the other and copy over the quantity. Add with others and done.

Not perfect, but might be enough for you.
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Christian
#40104192 120Gb (no longer in my E36 M3, won't fit the E46 M3)