If you buy an 8GB 3200MHz RAM, it will be electronically limited to 2666MHz
If you buy a 8GB 2400MHz RAM, the original 4GB RAM 2666MHz inside your PC will be downclocked to 2400MHz

Simplest answer: Buy an 8GB 2666MHz

PS: Ryzen processors perform better with dual channel RAM. And Dual Channel works best if both stick is the same size, same speed & same latencies. IE, buy 2x8GB 2666MHz then remove the original 4GB RAM

........ Did you not read what I wrote?
In order to get dual channel, the RAMs has to be identical

Examples:
You will get dual channel with these setup:
2 x 4GB DDR4 2666MHz
2 x 8GB DDR4 2666MHz
2 x 16GB DDR4 2666MHz

These setup will not get you dual channel:
1 x 4GB DDR4 2666Mhz + 1 x 8GB DDR4 2666Mhz
1 x 4GB DDR4 2666MHz + 1 x 16GB DDR4 2666MHz
1 x 8GB DDR4 2666Mhz + 1 x 16GB DDR4 2666MHz

Just sell the original stick and get 2 new 8GB DDR4 3000MHz
Problem solved

Don't expect a high price for your used RAM though. Max you can get it for probably around RM80