So even if he is 100 or 200 negative he is still having someone payoff his mortgage and within a year he could move to another and just keep doing this until he has filled up his 4 or 10 mortgage slots. 

You share the one hour slot with others and wait for your turn.

So that delivery people  (UPS, FedEx, etc) can drop them into the lockers and a key can be issued to the tenant (in their mailbox through a slot, for example).   

I would focus on that first and include a slot for the potential for an experienced partner or two in there.

The thing I see countless medical pros in my market doing is building a multiple-office building on the land, running their office out of the prime slot, then renting out the other offices to offset the debt service.Considering the "Happy Correlations" between dentistry and a few other industries the dentists in my area have had no shortage of "feeder businesses" wanting to rent the space, especially when it is shiny new construction :) 

See below:So my advice, get your 10 Fannie Mae loans, then refinance the bulk of those to a commercial blanket loan and open up your Fannie Mae slots again for more residential purchases with the best terms.

So calling it unit B is not going to help me one bit or for that matter the mailman.The owner next store took down the mailboxes as they're subject to vandalism, and have the mailman deliver the mail through the door slot.

My partner had slot of cash on hand and wants to invest, I have little to none to contribute but found a property, can serve as the property manager, am able to provide value based on my experience.

It let us drop our interest rate and free up loan slots as well.  

The mailbox is one of those where you just put it in a slot so it was never marked vacant.