Sum the value and find max

Hi, Anonymous 

Please check the below for creating a measure.

Buyer Measure =
IF (
ISFILTERED ( 'Table'[Part] ),
MAXX (
FILTER (
SUMMARIZE ( 'Table', 'Table'[Buyer], "@qtytotal", SUM ( 'Table'[QTY] ) ),
[@qtytotal]
= MAXX (
GROUPBY (
'Table',
'Table'[Buyer],
"@qty", SUMX ( CURRENTGROUP (), 'Table'[QTY] )
),
[@qty]
)
),
'Table'[Buyer]
)
)

Hi, My name is Jihwan Kim.

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Buyer Max = 
MAXX( TOPN( 1, DISTINCT( Sales[Buyer] ), CALCULATE( SUM( Sales[QTY] ) ) ), Sales[Buyer] )

bonus

Buyer Min = 
MAXX( TOPN( 1, DISTINCT( Sales[Buyer] ), CALCULATE( SUM( Sales[QTY] ) ), 1 ), Sales[Buyer] )