I'm not exactly sure what this means but I think it means that your monthly principal and interest don't factor into the equation.

If you have a mortgage, it'll get a bit tricky to enter the data (principal, interest, and escrow.)

You will then have a few options with that rent money....you can put it into principal or you can start saving it to purchase yet another property.

Do I just take my principal and interest and walk away?

We have around $300k in principal left, and I was thinking about a cash out refinance. 

Thanks  Pros:-Lower monthly payment = higher cash flows-Save more equity than paying down principal, can be utilized in other projects and to scale-Generally better for tax purposes (no tax benefit to paying down principal)Cons:-Higher rate-Higher risk (deleveraging over time give you more LTV cushion generally)-Less cash out / harder to refi in the future (UPB will be higher)

Both are passthrough entities, meaning the income and expenses ultimately flow to your personal return, and both have benefits and drawbacks.

My goal: create an interest-free loan which is appealing to a potential homebuyer, yet doesn't make too big of a discount on my end.For example: If I sold my $300,000 house with 20% down ($60,000) over 30 years, at 6.7% interest rate: $1548.67 principal and interest per month1548.67 x 12 x 30 = $557,521.20What I would do would offer a lower monthly payment and no usury, but it would effectively be like a prepayment penalty.Arbitrarily, let's say 20% off the monthly payment, or $309.73 less per month: $1238.94 monthly payment.House would be sold at $446,018.40, which is $111,502.8 less than the total paid with a normal mortgage, but $116,018 more than the market price.

I've read up on the benefits and drawbacks of real estate investing and have decided to take the leap forward towards that goal. 

You will then have a few options with that rent money....you can put it into principal or you can start saving it to purchase yet another property.