Please try
let
Source = Table.Combine({Trans, Cash}),
SelectColumns = Table.SelectColumns(Source,{ "Customer Name", "Date","Invoice", "Receipt", "Amount Incl VAT", "Amount"}),
DrCr = Table.RenameColumns(SelectColumns,{{"Amount Incl VAT", "Debit"}, {"Amount", "Credit"}}),
Sort = Table.Sort(DrCr,{{"Customer Name", Order.Ascending}, {"Date", Order.Ascending}}),
AIndex = Table.Buffer(Table.AddIndexColumn(Sort, "Index", 0, 1, Int64.Type)),
ABalance = Table.AddColumn(AIndex, "Balance", each
let f = Table.SelectRows(AIndex,(A)=> A[Customer Name] = [Customer Name] and A[Index]<=[Index])
in List.Sum(f[Debit]&{0}) -List.Sum(f[Credit]&{0})),
RemovedIndex = Table.RemoveColumns(ABalance,{"Index"})
in
RemovedIndex
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