Keep the document chain understandable
An estimate or proposal establishes the intended work. An invoice explains what is being charged, while payment and credit records show how the balance changes. Use consistent customer details, references and item descriptions across that chain. Where time or expenses feed the invoice, make the review point explicit. Repeat services need a considered schedule, including the treatment of changes, pauses and renewals.
Agree the boundary with accounting
A CRM can hold valuable commercial information without being the authority for every accounting decision. Establish which system owns final finance records and how invoices, receipts and corrections are exchanged. Test a representative billing period, including partial payments and credits, rather than checking only a straightforward sale. Tax treatment, financial reporting and filing requirements should follow your finance process and the exact supported accounting arrangement.