The final purchase price is always determined in fiat (government-backed currency).
For example, if the purchase price for the property you’re selling is $800,000, then the market value of the cryptocurrency (bitcoin, USDT, USDC, ETH, etc) will equal $800,000 at the time of closing.
The exchange rate is established based on the mutually agreed indicator from a publicly available exchange. You can reduce market volatility by choosing a stable coin whose value is already tied to USD, Euro, or other fiat currency.
If the transaction is completed as crypto to crypto, the specific instructions will be written into the purchase agreement or buy-sell contract or escrow agreement stating that the buyer and seller agree to the market fluctuations after the close of escrow and that the buyer is no longer responsible for those fluctuations.