Private loan, rental income, audit preparation

I've borrowed money from a family member for medical expenses.  I'm worried that we're going to get audited because there's a lot of money moving between our bank accounts as he sends me money and I slowly pay it back.  I know we don't owe taxes on private loans, but how should I prepare for the audit that I fear is coming?  Should we write something up explaining how much was loaned and what has been paid back?  How do I do this without making it look fishy?

Part 2:  The family member I borrowed money from is renting a house from me.  We have agreed that any amount less than the rent is to be applied towards what I owe.  So, if rent is $1,000/month and he gives me $600 one month, I am claiming $1,000 as income and paying taxes on that, but he is taking $400 off what I owe him.  Or, if he gives me zero dollars one month, I'm still claiming $1,000 as income and paying taxes on that, but he is taking $1,000 off what I owe him.  We are both fine with this arrangement, but I'm worried about that looming audit.  How should I prepare?  Should I write something up explaining this?  A spreadsheet, perhaps, showing what money was applied to what debt?

I want to be as transparent and prepared as possible.  Any help would be appreciated.  Thank you.