Query Running Balance

I am using the following query to create running balance at every change of
Invoice Number but no luck, here Invoice Number is a text field

SELECT [Inventory Movement Extended].InventoryCode, [Inventory Movement
Extended].InvoiceDate, [Inventory Movement Extended].InvoiceNumber,
[Inventory Movement Extended].[Company Name], [Inventory Movement
Extended].InventoryName, [Inventory Movement Extended].Purchase, [Inventory
Movement Extended].Sale, DSum("[Purchase]","Inventory Movement
Extended","[InvoiceNumber]='" & [InvoiceNumber] &
"'")-DSum("[Sale]","Inventory Movement Extended","[InvoiceNumber]='" &
[InvoiceNumber] & "'") AS Balance
FROM [Inventory Movement Extended];

any suggestions!!!

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Regards,

Abdul Shakeel