the sale is not final until closing. normally I would regard the down payment as a deposit but see the last paragraph below. the full sale will be reported in the year of closing. if you own and occupied the residence for any 2 years out of 5 years before the date of sale you get a $250,000 ($500,000 if married) exemption on the gain. there are partial exemptions available if you meet certain rules if the 2 out of 5 rules are not met. the exclusion applies if the home was never a rental while you owned it. if it was other rules apply. also I'm assuming the residence is in the US.
not sure if any potential buyer would make such a large down payment. there might be something involved that could make part or all of the $150K taxable in 2021. consult a lawyer.
