Using previous row in current row calculation

It appears that you want to add a "running balance" column to each "group" of customers.

You can do that in M Code by Grouping, then using a custom aggregation to create the new subtable.

Please read the code comments for more detailed explanation:

let
    Source = Excel.CurrentWorkbook(){[Name="Table35"]}[Content],
    typeIt=Table.TransformColumnTypes(Source, {
        {"Transaction ID", Int64.Type},{"Customer",Text.Type},{"Date",Date.Type},{"Net Change",Currency.Type}
    }),

//Group by Customer
    grp = Table.Group(typeIt,{"Customer"},{

//then add running balance column to each subtable
    {"runningBalance", (t)=> Table.FromColumns(
    Table.ToColumns(t) &
    {List.Generate(
        ()=>[rb=t[Net Change]{0}, idx = 0],
        each [idx]< Table.RowCount(t),
        each [rb = [rb]+ t[Net Change]{[idx]+1}, idx = [idx]+1],
        each [rb]
    )})}}),

//Remove original customer table
//Then expand the grouped table
    #"Removed Columns" = Table.RemoveColumns(grp,{"Customer"}),
    #"Expanded runningBalance" = Table.ExpandTableColumn(#"Removed Columns", "runningBalance", 
        {"Column1", "Column2", "Column3", "Column4", "Column5"}, 
        Table.ColumnNames(Source) & {"Outstanding"}),

//reset the data types
// could set up a more restricted environment than #shared if necessary
    types = List.Transform(Table.Schema(typeIt)[TypeName] & {"Currency.Type"}, each Expression.Evaluate(_,#shared)  ),
    reType = Table.TransformColumnTypes(#"Expanded runningBalance",List.Zip({Table.ColumnNames(#"Expanded runningBalance"),types}))
in
   reType