Prioritising return value according to column

Hi PELNZ_kjm 

Here is one way

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WciwuTi0xVNJRMjDUNzDTNzIwAnEcgdjQQClWB5cCJ5ACUzQFRugKjBAKjLBZYWSMpsAIXQGSG4zRTXAGYlMkBSboClyA2ByoIBYA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ASSET = _t, DATE = _t, SOURCE = _t, VALUE = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ASSET", type text}, {"DATE", type date}, {"SOURCE", type text}, {"VALUE", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"ASSET", "DATE"}, {{"allrows", each _, type table }}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each if List.ContainsAll( [allrows][SOURCE],{"A","B"}) then Table.SelectRows([allrows], each [SOURCE] <>"B") else [allrows]),
    #"Removed Other Columns" = Table.SelectColumns(#"Added Custom",{"Custom"}),
    #"Expanded Custom" = Table.ExpandTableColumn(#"Removed Other Columns", "Custom", {"ASSET", "DATE", "SOURCE", "VALUE"}, {"ASSET", "DATE", "SOURCE", "VALUE"})
in
    #"Expanded Custom"