If doing it the old fashion way, you could always do an addendum stating the terms of your payment arrangement.

Of course, there are ways to skirt this (namely through appointment-style visits arranged through free online services like Craigslist).

There are countless ways to structure these types of partnerships, and it should be a mutually beneficial  arrangement - make sure you find someone who doesn't only have their own pockets in mind. 

How was the arrangement "documented" as you state?

We could arrange to meet up in person.

So you can arrange this in any way that's agreeable to both parties.I've done it from both sides, though through a broker in all cases. 

I can't really say what I would give on a cash for keys, without much more background info.I do think if you can arrange a face to face, regardless of whether you want to rent to current occupant or not, you will learn if you want to take the risk of dealing with him.

I prefer using their money without the partnership arrangement.

The most common question people will have is “What are the sleeping arrangements?”

Additionally, partnership tax law for the more complex operating arrangements is often beyond the professional expertise of most tax preparers including most general tax practice CPAs.