you send $1M of btc to their wallet
...
It's not a downpayment and there's no crypto sale.
In the U.S., I think it is a crypto sale because the IRS treats crypto as property.
https://www.forbes.com/sites/robertwood/2021/08/02/three-tax-free-crypto-transfers/?sh=40a1bcdfb3f7
https://www.irs.gov/pub/irs-drop/n-14-21.pdfThe tax paradox started when the IRS ruled that cryptocurrency is property in Notice 2014-21.
Q-1: How is virtual currency treated for federal tax purposes? A-1: For federal tax purposes, virtual currency is treated as property. General tax principles applicable to property transactions apply to transactions using virtual currency.
That doesn't mean the idea won't work, but that ruling makes it a little more complicated for both sides.