Hi,
You should start with the Google Sheets “Search” module, which will search for the current month entry. Then set up mapping to the email module or the Gmail module if you are using Gmail.
It is a pretty straightforward scenario.
The tricky part is setting up the scheduling, which will be a crucial part of your project. Here is your solution: Schedule Scenarios to Run on the Last Day of the Month.
Bonus if we can also add a trigger that will send him an automatic email receipt of his payment with current payment, method of payment, remaining balance, etc. He typically pays in cash, which is why this process is such a pain.
I don’t know what your local law requires, but don’t you need a signed document? If so, you can achieve it with the Google Docs create from template module. If email is enough- you should learn more about webhooks here
Good luck!
Have a nice day, Michal