Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (743.99 USD): od7off
Coupon (790.99 USD): 7bcdf9
(CouponsFromChina.com tip: just apply this code in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGM9P10, BGBFAFF2019, BGCITYADS2019
(there’s a little possibility that old coupon will work)
Price history and price alert
Check out price history chart (for all/most countries) for Xiaomi Mi9 Pro 5G 12/512GB Black.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).