The case is built before it is filed
By the time a dispute with a business partner reaches a courtroom, the outcome is largely fixed by the record that already exists: the contracts, the emails, the minutes, the payments. Courts decide on evidence, not on who feels wronged, and Art. 8 CC puts the burden of proving a fact on whoever derives a right from it. The weeks before proceedings are therefore not a waiting period, they are when the case is actually made.
Start by securing everything relevant while you still have access: the agreements and their amendments, correspondence, board and shareholder minutes, invoices and accounting records. If your partner controls shared systems or the company's files, secure copies now, lawfully: access has a way of disappearing once the dispute is open. Write a chronology while memories are fresh; it will discipline both the legal analysis and your negotiating position.
Quantify, precisely
"He owes me a lot" is not a claim. Work out exactly what you are claiming, on what legal basis, and how each amount is evidenced. The exercise is sobering and useful: it usually shows that part of the claim is strong and part is speculative, which changes both the demand and the realistic settlement range.
Just as important is the other side of the ledger. Counterclaims are standard in partner disputes; their likely size belongs in your assessment from day one, not as a surprise in the answer to your lawsuit.
The formal demand, and two honest assessments
Send a formal written demand: the facts, the basis, the amount, a deadline, and what follows if it passes unheeded. It also puts the partner in default (Art. 102 CO), and where you are waiting on performance rather than money, the deadline you set is what later opens the choices in Art. 107 CO. A precise demand puts the dispute on record, stops drift, and tests whether the matter can settle before costs escalate.