By starting at the original event in each case. A share buy-back is proved by the agreement, the corporate resolutions, the valuation and the instalment payments as they appear in bank records. A matrimonial division is proved by the judgment or notarial act and the transfers that carried it out.
Two chains, each beginning at a document somebody official signed
Hypothetical example: Solveig's money comes from two places, an instalment buy-back of her half of a company and a matrimonial division settled in court. Treat the sources separately and complete each chain before combining them. For the buy-back, the file needs the executed agreement, the resolutions authorizing it, the valuation or price mechanism, the company's financial statements for the relevant years, evidence that Solveig lawfully held the interest in the first place, and bank records showing each instalment arriving.
Her tax filings for those years should be consistent with what the agreement says she received. For the matrimonial division, the file needs the judgment or the notarial act, any accompanying schedule of assets, and the transfers or transfer instruments that gave effect to it. Documents drawn in French require complete certified translation.
Two habits cause avoidable trouble. Money that moved through a spouse's account or a family member's account creates a gap that has to be explained with its own documents. And cash instalments recorded only as deposits, without a matching contractual entitlement, are the single most common place these files stall.