Mortgage Calculations With A Difference

Scenario - I started doodling with buying a property to let and then started thinking about adding the "shortfall" between the required bond payment and the income generated by the property back onto the loan each month as the shortfall arises. Let's assume that the financial institution will "pay" this shortfall from a previously agreed upon additional amount available on the property - understand?

While there is a shortfall between the income generated by the property and the bond payment, insurance, taxes etc., the outstanding capital will obviously increase as each new month arises and each specific shortfall is added onto the outstanding capital (loan at the bank).

Once the shortfall turns into surplusses, the surplusses are in turn paid into the account until the bond is paid in full.

With a normal, say 15 year, bond, the interest payments and principal payments of the constant mortgage payment (assume fixed interest rate) nicely works out to a balance of 0 in month 180, but not in the scenario I describe.

What happens in the scenario I described above is that, if I keep the payment constant (=PMT(and the rest - calculated on the original term and the original investment) each month's deficit is added to the outstanding capital but the payment stays constant. I want this payment to remain constant throughout the bond.........