Connecting two finance systems is often demonstrated with a green tick. The tick means a request succeeded. It does not mean the books agree. An invoice can exist in the accounting file and not in the collection record, or the other way around. A payment can be applied in one and still open in the other. A customer can have two spellings and one bank account.
Firms feel this more than anyone, because they already close in Tally, QuickBooks, Xero, Zoho Books, or, further up the market, SAP and NetSuite. They do not want a new ledger. They want the chase to stop contradicting the file they sign.
Write down the difference
A grown-up sync keeps a variance list. What is in Zen and not in the file. What is in the file and not in Zen. What is in both, with amounts that do not match. That list is the work. Hiding it behind a successful sync is how month-end becomes archaeology.
Direction matters too. Some facts should flow from the accounting file, such as the customer you already trust. Some facts should flow back, such as a payment received on a link. Neither direction should overwrite a closed document because a later edit arrived. A correction is a new event.
Neighbours, not replacements
CRM, ERP, and payroll systems know things a billing record needs: which account a customer belongs to, which entity employs them, which contract is live. Reading those facts is reasonable. Becoming the CRM, or running payroll, is a different product. The API and the webhooks should carry billing events, not a promise that every neighbouring system can now be retired.
If you evaluate a connection, ask to see a deliberate disagreement. Change a total on one side and watch whether the other side notices. A demo that only shows the happy path has not shown you a sync. It has shown you a copy.