New to manager and need help for my business

Hello everyone. I am new to manager and I am having some problems. I’m gonna explain my business in a nutshell and hopefully you guys can help me.

My business: I lend money to people and charge a daily interest on it. Let’s say you are my customer and I gave you 5000 bucks. I will charge a daily interest of 18% on it. For example, you have it for 215 days, the total amount you have to pay me will be, according to simple interest, 5000*(18/100)*(215/360) = 537.5
But there’s another thing. Suppose you only have 2000 at this time and you pay it to me. I will not subtract the 2000 from 5000, but instead, I will add interest on these 2000 bucks. So these 2 will go as 2 separate values. So at one hand I’m adding interest to the 5000 I gave you, and on the other hand, you are adding interest on the 2000 you gave me.

It’s kinda hard to understand but if you got it, please help me. Thanks!

So, what is the question here?

How to automate calculation? If so, it is not possible with Manager. You should use some other software for this (e.g. spreadsheets - Excel, LibreOffice etc).
How to post transactions?
Or what accounts should be in charter of accounts?

Thanks for the reply. I experimented a littler with Manager and found out that I can do the loan thing with sales invoices. But there is no way of adding interest to the customer payments. I was hoping you could tell me if there is a way I can do this. And if not, can you suggest a software that can help me?

You could issue credit notes, then the interest payable to the customer would reduce the interest payable to you.

But overall it would be rather simpler to charge interest for the period of 5000 and then charge interest for the period of 3000 as the interest is only going one way, unless the interest payable on the 2000 is at a lower rate then the interest being charged on the 5000

Thank you