Add receipt field to payments

[deployn]

Is your feature request related to a problem? Please describe.
There are times my business gets payments without sending an invoice beforehand or afterwards. I would like to add a receipt for documentation to those.

Describe the solution you'd like
A receipt field like in expenses which holds a PDF or an Image file of the payment.

Describe alternatives you've considered
The alternative to create an invoice isn't working, an invoice doesn't contain a receipt field.
The alternative to create an expense isn't working, the total can't be below 0.