DOS, mit seinen paar Batch-Befehlen kann das wirklich nicht. Aber dafür gibt's ja den Windows Scripting Host. Leg eine Textdatei an und gib ihr die Endung .vbs. Der Inhalt des Visual Basic Scripts könnte in etwa so aussehen:
' Read data from "C:\tmp\Rechnungen.txt" and split by client
Option Explicit
On Error Resume Next
Const ReadFile = 1, WriteFile = 2
Dim fso, BillFile, TextStream, TextLine, BillName, NewBill
Set fso = CreateObject("Scripting.FileSystemObject")
Set BillFile = fso.GetFile("C:\tmp\Rechnungen.txt") ' complete file
Set TextStream = BillFile.OpenAsTextStream(ReadFile)
Do ' get first bill number
TextLine = TextStream.ReadLine
Loop Until TextStream.AtEndOfStream Or _
Left(TextLine, 1) = "f" And IsNumeric(Mid(TextLine, 2, 6))
Do Until TextStream.AtEndOfStream
BillName = Left(TextLine, Instr(1, TextLine, "=") - 1) & ".txt"
Set NewBill = fso.OpenTextFile("C:\tmp\" & BillName, WriteFile, True)
Do ' write single client bill file
 NewBill.WriteLine TextLine
 TextLine = TextStream.ReadLine
Loop Until TextStream.AtEndOfStream Or _
 Left(TextLine, 1) = "f" And IsNumeric(Mid(TextLine, 2, 6))
NewBill.Close
Loop
TextStream.Close
Set NewBill = Nothing
Set TextStream = Nothing
Set BillFile = Nothing
Set fso = Nothing