"Remaing Balance" through Custome Report along with "Custom Field"

I Generate the Custom Field in Sales Invoice
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Found result as per requirement

Sales Invoice Report by Custom Field

Custom Report for Specific Custom Field

Result
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Now the Question is What should i add to have Remaining Balance in Invoice

@lubos Please any solution…?

A custom solution will only be available once the Extentions part has been finalized. But for now, that shouldn’t be achievable.