Currently, you do not have the ability to generate an invoice to send to a client within MassageBook. A receipt is generated only when a sale is recorded during the checkout process.
There are plenty of free invoice generator programs available online if you need to send one to a client. A quick online search should yield good results.
Once you receive payment from the client, you can check out the client's appointment on the Schedule to record the sale. Also, you can add a reminder to the client's record that the client still owes you for a past session. See the following article for more info about adding notes and reminders: Adding Notes to a Client's Record