Identify used batch index numbers AND split up and add records for each batch

I made it work, but there are some prerequisites - both tables need to have a unique index and Cumulative KG column, like this:

Function for getting the first receipt that has cumulative value bigger than cumulative demand. It has to be named 'giveReceipt'

giveReceipt

(value as number) => let
    Source = #"Material Receipts",
    #"Removed Other Columns" = Table.SelectColumns(Source,{"Index", "CumulativeR"}),
    FirstBigger = Table.SelectRows(#"Removed Other Columns", each [CumulativeR] >= value ),
    Max = List.Min(FirstBigger[Index])
in
    Max

Material Receipts

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjTQNdQ1MjAyUNJRMjIAAiBdEpSanJpZUFIMZBsqxeoAVZkiVBmam6GpMQKrMdY1xmeQsVJsLAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Date = _t, KG = _t, Source = _t, Index = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type date}, {"KG", Int64.Type}, {"Source", type text}, {"Index", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "CumulativeR", each List.Sum(List.Range(#"Changed Type"[KG],0,[Index])))
in
    #"Added Custom"

Material Demand

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bcxLCoAwDEXRvWTclCRtja7DYelIBEX8gN0/FgpSxNm7cHgxAnsUFBICA8jKNmgoM4/Leu3zke8SDMkUqa0UtkxV5nPaXiWE7lVC3KsdVL+PUm1obCDf/VEHKT0=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Date = _t, KG = _t, Source = _t, #"Source Index" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type date}, {"KG", type number}, {"Source", type text}, {"Source Index", Int64.Type}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1),
    AddedCumulative = Table.AddColumn(#"Added Index", "Cumulative", each List.Sum(List.Range(#"Changed Type"[KG],0,[Index]+1))),
    #"Changed Type1" = Table.TransformColumnTypes(AddedCumulative,{{"Cumulative", type number}}),
    AddedFirstBigger = Table.AddColumn(#"Changed Type1", "Max", each giveReceipt(-[Cumulative])),
    Custom2 = Table.FromList( List.Combine({{1},List.RemoveLastN(AddedFirstBigger[Max],1)}), Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    Custom1 = Table.FromColumns(Table.ToColumns(AddedFirstBigger)&Table.ToColumns(Custom2), Table.ColumnNames(AddedFirstBigger)&{"Min"}),
    #"Added Custom" = Table.AddColumn(Custom1, "Receipt", each {[Min]..[Max]}),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Max", "Min"}),
    #"Expanded Receipt" = Table.ExpandListColumn(#"Removed Columns", "Receipt"),
    #"Merged Queries" = Table.NestedJoin(#"Expanded Receipt", {"Receipt"}, #"Material Receipts", {"Index"}, "Material Reciepts", JoinKind.LeftOuter),
    #"Expanded Material Reciepts" = Table.ExpandTableColumn(#"Merged Queries", "Material Reciepts", {"KG", "CumulativeR"}, {"Receipt.KG", "Receipt.CumulativeR"}),
    #"Added Custom1" = Table.AddColumn(#"Expanded Material Reciepts", "Custom", each [Cumulative]+[Receipt.CumulativeR])
in
    #"Added Custom1"

there is lot of dependencies between the tables so it may be good if you try to see it in a new file first. It's not giving the final state that you needed, but has all the details to get there:

e.g. Custom should replace negatives with 0, etc.

Here is another solution

let
    processDemand = (batchIndex as number, recieptIndex as number, usedFromBatch as nullable number, remainingFromReciept as nullable number) =>
        let
            isNewBatch = newUsedFromBatch = 0,
            isNewReciept = newRemainingFromReciept = 0, 
            currentReciept = #"Material Reciepts"{recieptIndex}?,
            currentDemand = #"Material Demand"{batchIndex}?, 
            newUsedFromBatch = List.Min({remainingFromReciept + usedFromBatch, 0}),
            newRemainingFromReciept = List.Max({remainingFromReciept + usedFromBatch, 0})
        in
            if currentReciept = null or currentDemand = null then
                {}
            else
                {currentDemand & [Receipt Index = recieptIndex, Receipt KG = #"Material Reciepts"{recieptIndex}[KG], Used from Batch = usedFromBatch, Remaining from Batch = newRemainingFromReciept]} &
                (
                    let
                        batchIndex = if isNewBatch then batchIndex + 1 else batchIndex,
                        newUsedFromBatch = if isNewBatch then #"Material Demand"{batchIndex}?[KG]? else newUsedFromBatch,
                        recieptIndex = if isNewReciept then recieptIndex + 1 else recieptIndex,
                        newRemainingFromReciept = if isNewReciept then #"Material Reciepts"{recieptIndex}?[KG]? else newRemainingFromReciept
                    in
                        @processDemand(batchIndex, recieptIndex, newUsedFromBatch, newRemainingFromReciept)
                )
in
    Table.FromRecords(processDemand(0, 0, #"Material Demand"{0}[KG], #"Material Reciepts"{0}[KG]), type table [Date = date, KG = number, Source = text, Source Index = Int64.Type, Receipt Index = Int64.Type, Receipt KG = number, Used from Batch = number, Remaining from Batch = number])