Mortgage Payoff question

[Ron Tremblay]

I have a private mortgage from my parents, 2 years ago, I "prepaid" 12 months of it with a bonus I recieved from work, in order to free up monthly cash flow to rent a (much larger) apartment, also from my parents. I moved out of the house I owned, and rented that out to a tenant, and moved into the apartment my parents own. Now, 2 years later, I am selling the house I own, and we (my parents and I) are both trying to figure out what the amount is to pay off the mortgage. Since I paid 12 months at once, does it have any effect on interest owed? or do we just treat it like I paid those 12 months on time?

Some facts and numbers

Purchased house on Dec 31st, 2014.

Original loan  of 100,000, 4% interest

PITI payment 743.64- we re-evaluated actual taxes and insurance cost every Jan, and I wrote an extra check for the difference each year.

Each payment was direct deposited into my parent's acct on the 1st of each month. never late.

12/1/18 when I paid 8923.68, for Dec of '18 thru Nov of '19 

wrote check for difference in taxes and insurance jan 1 '19

12/1/19 started regular payments again.

I have an excel spreadsheet with my amortization schedule, and with no modifications, it says that on 11/1/20 i had a balance of 88661.28. If I change the extra payment on 12/1/18 to 8180.04 (11 months), and change the principal paid on 1/1/19 thru 11/1/19 to 0, I get a balance of 85388.21. 

Does that seem correct?

Thanks for the input!