Evidence of how the partnership interest was originally acquired, the buyout agreement and its consideration, the sawmill's financial and tax records supporting the value, and a complete banking trail for every tranche from the payers to the enterprise.
Every tranche needs its own chain of custody
Treat each payment as a separate case. For the first instalment, the record shows the partners' funds leaving their accounts, arriving in Grigor's, and moving on to the enterprise, with dates that line up and no unexplained intermediate stops. Repeat that for every subsequent payment.
Sitting underneath is the older history: how he came to own the interest, what he paid or contributed, and how the mill's filings reflect the ownership over the years. Where a document is in French or another language, a certified English translation is required. Informality is the usual problem in long-running partnerships, where interests shifted by agreement and were recorded in minutes if at all.
Reconstructing that history takes months and is best started before, not after, an adviser is asked whether the case is viable.