Этот код должен работать:
let
Source = Excel.CurrentWorkbook(){[Name="Data"]}[Content],
group = Table.Group(Source, {"TopPaymendId"}, {"TopAmount", each List.Sum([JournalLineNetAmount])}),
join = Table.Join(Source,{"TopPaymendId"},group,{"TopPaymendId"}),
replace = Table.ReplaceValue(join,each [TopAmount],each if [TopPaymendId] = null
then [JournalLineNetAmount] else [TopAmount],Replacer.ReplaceValue,{"TopAmount"})
in
replace