Convert transactions to balance

An advanced Power Query solution below, creating a record for each possible combination SKU/Warehouse/Date with quantities and balances.

It is assumed that your data has unique SKU/Warehouse/Date combinations (i.e. max 1 transaction per SKU / Warehouse / Date), otherwise a grouping step must be added.

If your data set is large, you should expect quite some runtime, but I already managed to reduce runtime by adding helper columns PrevSKU and PrevWarehouse with the values from the previous rows: Power Query is much faster when values on 1 row can be compared with each other, rather than values on different rows,

let
    Source = Excel.CurrentWorkbook(){[Name="Transactions"]}[Content],
    Typed1 = Table.TransformColumnTypes(Source,{{"Date", type date}, {"Warehouse", type text}, {"SKU", Int64.Type}, {"Qty", Int64.Type}}),
    UniqueSKUs = Table.Distinct(Typed1, {"SKU"}),
    RemovedOthers = Table.SelectColumns(UniqueSKUs,{"SKU"}),
    WarehouseLists = Table.AddColumn(RemovedOthers, "Warehouse", each List.Distinct(Table.Column(Typed1,"Warehouse"))),
    ExpandedWarehouse = Table.ExpandListColumn(WarehouseLists, "Warehouse"),
    DateLists = Table.AddColumn(ExpandedWarehouse, "Date", each List.Distinct(Table.Column(Typed1,"Date"))),
    ExpandedDate = Table.ExpandListColumn(DateLists, "Date"),
    Merged = Table.NestedJoin(ExpandedDate,{"SKU", "Warehouse", "Date"},Typed1,{"SKU", "Warehouse", "Date"},"NewColumn",JoinKind.LeftOuter),
    ExpandedQty = Table.ExpandTableColumn(Merged, "NewColumn", {"Qty"}, {"Qty"}),
    ReplacedNullsWithZero = Table.ReplaceValue(ExpandedQty,null,0,Replacer.ReplaceValue,{"Qty"}),
    SortedSKUWHSDate = Table.Sort(ReplacedNullsWithZero,{"SKU", "Warehouse", "Date"}),
    SKUWarehouse = Table.SelectColumns(SortedSKUWHSDate,{"SKU", "Warehouse"}),
    PrevSKUWarehouse = Table.InsertRows(SKUWarehouse,0,{[SKU = null,Warehouse = null]}),
    AddedPrevSKUWarehouse = Table.FromColumns({Table.ToRecords(SortedSKUWHSDate),Table.ToRecords(PrevSKUWarehouse)}),
    Expanded1 = Table.ExpandRecordColumn(AddedPrevSKUWarehouse, "Column1", {"SKU", "Warehouse", "Date", "Qty"}, {"SKU", "Warehouse", "Date", "Qty"}),
    Expanded2 = Table.ExpandRecordColumn(Expanded1, "Column2", {"SKU", "Warehouse"}, {"Column2.SKU", "Column2.Warehouse"}),
    RenamedColumns1 = Table.RenameColumns(Expanded2,{{"Column2.SKU", "PrevSKU"}, {"Column2.Warehouse", "PrevWarehouse"}}),
    RemovedBottomRow = Table.Buffer(Table.RemoveLastN(RenamedColumns1,1)),
    Balance = List.Generate(
                () => [Counter = 0, Balance = RemovedBottomRow[Qty]{0}],
                each [Counter] < Table.RowCount(RemovedBottomRow),
                each [Counter = [Counter] + 1, Balance = if RemovedBottomRow[SKU]{Counter} = RemovedBottomRow[PrevSKU]{Counter} and 
                                                            RemovedBottomRow[Warehouse]{Counter} = RemovedBottomRow[PrevWarehouse]{Counter}
                                                         then [Balance] + RemovedBottomRow[Qty]{Counter}
                                                         else RemovedBottomRow[Qty]{Counter}],
                each [Balance]),
    AddedBalance = Table.FromColumns({Table.ToRecords(RemovedBottomRow),Balance}),
    Expanded3 = Table.ExpandRecordColumn(AddedBalance, "Column1", {"SKU", "Warehouse", "Date", "Qty", "PrevSKU", "PrevWarehouse"}, {"SKU", "Warehouse", "Date", "Qty", "PrevSKU", "PrevWarehouse"}),
    RenamedColumns2 = Table.RenameColumns(Expanded3,{{"Column2", "Balance"}}),
    RemovedColumns = Table.RemoveColumns(RenamedColumns2,{"PrevSKU", "PrevWarehouse"}),
    Typed2 = Table.TransformColumnTypes(RemovedColumns,{{"Warehouse", type text}, {"Date", type date}, {"Balance", Int64.Type}})
in
    Typed2