Invoicing for Multiple Clients (Rechnungen für mehrere Kunden): How a Treuhandbüro Keeps Every Mandate Straight

A Treuhandbüro rarely bills only for itself. The core of the trade is the mandate: you send the invoices of dozens of client companies, in their name and under their UID. Multi-client invoicing is a discipline of its own — who bills, under which VAT scheme, in whose numbering circle, and who keeps the records.

The mandate is the legal basis for every invoice

Every engagement rests on the mandate contract. Under Art. 394 of the Code of Obligations, a mandate is a contract whereby the mandatee undertakes to conduct certain business in accordance with its terms.[1] The client remains the Rechnungssteller; your office acts for them.

Billing in the client’s name is ordinary mandate business: the scope includes the authority to carry out such transactions as are required for performance, as Art. 396 puts it.[1] Even so, put the details in an engagement letter: who issues, who sends, who duns.

Care has a legal price. The mandatee is liable to the mandator for the diligent and faithful performance of the business entrusted to him, says Art. 398.[1] A wrong VAT rate on a client invoice is your error, not the client’s.

In practice, no invoice leaves the office before the signed mandate and a current power of attorney (Vollmacht) are on file. Review both yearly; mandates and signing rules evolve.

What you need per client before the first invoice goes out

Start with identity: the legal name exactly as registered, the registered office and the UID. Do not copy these from old letterheads — look the client up in the official UID register.[3]

Then the VAT profile. Liability follows each client separately: one stays exempt below the CHF 100,000 turnover threshold, others apply standard or reduced rates, a few use the Saldosteuersatz.[2] The scheme belongs to the client — applying it correctly is your job.

Numbering runs per legal entity. Each mandate keeps its own Rechnungsnummern circle, continuing exactly where the predecessor stopped — no gaps, no overlaps. The client’s auditors will ask.

Finally the payment rail: the client’s IBAN or QR-IBAN, payment terms, invoice language and eBill use. The QR-bill payment part carries the client’s account, never your own.[4]

The monthly run: billing a portfolio without mixing mandates

Organise by mandate, not by calendar date. Every client gets a fixed billing cycle with a defined cut-off, and you finish one mandate — draft, check, send, file — before starting the next.

Check every draft against the client’s master data: name and UID from the register, the right VAT treatment, the next number in the circle, the client’s account on the QR-bill. Give the first invoice of each new mandate a four-eyes review.

Delivery follows the mandate too: the document belongs to the client, so it goes out under their branding and, where agreed, from their channels. Your office signs the covering note, not the invoice.

Receivables stay with the client: payments land on their account and you keep the offene Posten list per mandate. When an invoice ages you propose the escalation — Zahlungserinnerung, Mahnung, ultimately Betreibung — but the decision to enforce is the client’s.

Your own duties: records, secrecy and oversight

Records are kept per mandate. Accounting records and vouchers must be retained for ten years starting at the end of the financial year[1] — keep them separated by client and retrievable at any time.

Client financial data is sensitive personal data. The revised Data Protection Act expects appropriate technical and organisational safeguards,[5] so grant access per mandate, apply least privilege and know where your tools host data.

Professional standards sit on top: many fiduciary firms join a self-regulatory organisation under FINMA supervision, which imposes its own conduct and secrecy rules on members.[6]

Get these rules right and the tooling is the easy part: facturio keeps contacts, VAT settings, numbering and QR-bills per client, so every invoice leaves the office attributable to exactly one mandate. Sources: [1] Code of Obligations (fedlex); [2] ESTV, VAT tax liability; [3] UID register; [4] SIX, QR-bill standard; [5] FDPIC, new Data Protection Act; [6] FINMA, self-regulatory organisations.