If a customer paid you in cash and you did not deposit the cash into a connected bank account (so it will not appear in your Ledger), you can still record the payment in Ambrook.
How to mark the invoice as paid
Open the invoice and confirm the details
Click Save as Sent
Click the three dots in the top right and select Mark as Paid
Choose Transaction
Select your Cash on Hand account.
Enter the payment date and amount
Click Save
The invoice will now show as Paid and it will be matched to a transaction recorded on your Cash on Hand account.
If the cash was deposited into a connected account
If you deposited the cash into a connected bank account, the deposit should appear in your Ledger. In this case, match the deposit transaction to the invoice instead of creating a manual cash transaction.