okay so here's the problem:

Mr. Go borrowed from Ms. Sy a certain amount payable at 9% compounded monthly after three years. Ms. Sy required Mr. Go to make regular payments of $9,000 at the end of each month to settle his obligation. Everything went as agreed upon for a year. After the first year, however, Mr. Go missed remitting his monthly payments for five months. In order to make up for his missed payments, Mr. Go paid $60,000 a month after the fifth month of missed payments and made regular installments of $9,000 at the end of each month only for six months. After which he decided to pay Ms. Sy the rest of his loan through three quarterly payments of $40,000 each.

At the end of the three-year loan term, Ms. Sy claims that Mr. Go has not adequately paid the principal and all accumulated interests on the loan. Mr. Go, on the other hand, maintains that he has paid all his dues and has no obligation left unpaid. He adds that he, in fact, has paid more than he was supposed to.

how much does Mr. Go either still owe or has overpaid Ms. Sy (if any)?

for the reference purposes, the PV =283021.2473and FV =370374.4451

Here's the work I did in excel using the method of Outstanding Balance

this work shows if Mr. Go had not missed any payments

here's the work on Mr. Go's actual payments. At the end it shows that he has overpaid by $18235.483, pretty simple right?

I thought about the perspective of Ms. Sy, however, and used the sinking fund method to double check, the table below is in exact sync with the first table given that Mr. Go had not compromised his payments.

so far, it makes total sense, but then when I changed the table to fit the actual payments of Mr. Go, it all goes wrong...

the table then shows that Mr. Go falls short of his payments.

Can anyone help shed light into this?

I may have overdone it so tell me where I went wrong with my line of thought. Much appreciation to all of you.