Interest payment of loan

My father in law help to pay part of my husband morgate 15 years ago. He asked my husband and his wife at the time (now divorce) to sign a statement that they will have to have the principal in 30 years time, with interest of 4% per annum to be paid monthly. He said the money was a gift but the need the statement for his business tax purpose. My father in law never said anything about this money until last week when they got into an argument. My father in law wanted payment for the money plus 15 years of interest.
Actually when I read the agreement there is a clause that stated the interest have to be paid monthly, and even if the interest is not paid for one month, the loan has to be paid in full directly.
It has been 15 years my husband totally forgotten about this money, and my father in law never said anything about interest payment even once during these years. Are we legally bind to pay the 15 years interest, or can we just pay the initial principal?
We are located in Japan.

If you can show it was a gift or became a gift then NOTHING is due.

Build up that argument with any and all evidence that you have--witnesses too.

If the agreement contained the 4% interest per month, that is how it is to be paid.

The agreement you explain is enforceable and as such, the guarantor can take the guarantee to court and win a judgment for the money owed.

It's not a gift. As acknowledged in your post, a written contractual agreement was signed. That proves the money was not a gift, but a loan.