You have 1,000,000 outstanding now at 10% interest.
So R8,333 in interest every month.
Let’s say your original instalment was R10,000 so only R1668 actually goes towards paying the loan off.
Now you put in your 700k and therefore your outstanding balance is 300k.
So now interest is R2500 per month and R7500 goes towards actually paying off the capital balance.
If you really wanted to have your monthly instalment go down you can put the 700k in and ask FNB to balance the account. But now you’ll lose access to that 700k and you’ll still pay your house for 20 years at basically the same interest rate meaning you lose out over the term.
You want to pay it off faster if you want to save money, not pay it longer for less.