How much do i owe the IRS on Sept 15th for income received in September?

You might not have to pay them anything until April 2021.

The key is to make sure that you meet the IRS safe-harbor rules. If you make sure that through withholding and quarterly payments you have submitted to the IRS 110% of last years taxes, then you skip any penalties and interest related to this big capital gain.

I did this the last time I sold a house. It was a rental, and I couldn't get a close estimate of the impact with depreciation, recapture, and capital gains. So I made sure I had enough withheld from my job to make the safe-harbor. Then I sent the balance the following April.

You will have to see what California requires.