To issue an invoice to a client this can be done by going to the POS menu, select Receipts and choose the receipt that you want to issue the invoice.
Click long on the receipt and select: Email invoice or Print Invoice.
After this, you will need to select the customer for whom you are issuing the invoice.
If the Customer already exists in your Client application, you will need to search for its name and select it from the drop down list.
If you choose to send the invoice via E-mail, you will need to click Yes, to send it.
If you choose to Print the Invoice, you just need to select the client.
If the Customer does not exist, you will need to create it, from the + button.
For creating a new Client in POS please Click Here
Edge cases
I can no longer find the receipt in POS to send the invoice
The POS only displays receipts from the last 6 days. For an older receipt, issue or send the invoice from the admin account:
Go to the Sales menu in admin.
Search for the receipt using the magnifier — you can filter by date and by payment method.
Open the order (3 dots → View) and issue / send the invoice.
Note: a modern payment on a Z report represents a bank transfer. If the receipt is not older than 6 days, you can also find it in POS → Receipts, filtering by payment method.