Agree and it is the key to understanding how best to used Manager. A test company with a single transaction is the easiest way of seeing what it does and if it is the easiest way of achieving what you want. At the start it is worth trying several options to find the one you like the best.
I suggest you read the table of contents for the guides. It will give you an overview of what information is readily available. Then look in detail at particular guides which sound relevant to your current problem.
For a cash sale see https://www.manager.io/guides/7821
- enter a payment
- select contact → Customer → the customer
- Enter line items depending on the details of what happened (physical product returned in new sale condition → probably to sales account to reverse the prior sale, vs returned item not re-saleable → cost of sales account)
For invoice sale see https://www.manager.io/guides/5495
This thread may help About cash advance for employees - #3 by Abeiku
I suspect that is done via payslip items, see
Payslips: Set up payslip items | Issue payslips | Set up and manage recurring payslips
By the way, I find setting up a recurrent payslip for each employee valuable. Enter what ever is constant for each employee (items & rates) the add the extra details when each payslip is generated.