I amended a sales invoice 599/1. the next invoice should have been 650 but its going to 599/2.?

Great day, @Dg2.

Thanks for posting here in the Community. I'm here to help share information about how invoice/sales forms numbering works in QuickBooks Online (QBO).

When creating an invoice/sales forms, QB assigns the transaction numbers automatically to help you identify transactions easily. Since you have amended a sales invoice, the next entry will automatically follow the number in sequence with the last invoice. You can also reset your transaction numbers to suppress this activity. Here's how:

1. Click the Custom transaction numbers checkbox, then Save.
2. Create a transaction with a new starting number.
3. Clear the Custom transaction numbers checkbox, then click Save.

For more information, you can also refer to our help article: Change the number on invoices or other transactions. It contains guides on how to turn custom transaction numbers ON or OFF, changing invoice or sales numbers and how to reset it.

Feel free to visit us again if you need help with anything else. We're always delighted to assist.