Hi,

Before providing steps to correct the mistake, let me ask you a few questions:

1. Are you talking about Sales Invoice or Purchase Invoice?
2. Are you using On Account or Prepayments? You can check this in module Sales Ledger >> Settings >> Account Usage S/L >> checkbox Use Prepayments, not On Account - is it ticked or no?
3. Is checkbox Force Unique Prepayment Numbers ticked in module Sales Ledger >> Settings >> Account Usage S/L?

This information will help us, thank you.

Kind Regards,
Pijus
HansaWorld Support