Why would China sell weapons to Russia?

China: "Send the rest of your soldiers and weapons to the front with Ukraine."

Russia: "You're asking us to leave virtually all of our country without defense, while your army sits on our borders?"

China: No, no, our army will take your positions and defend your territory as if it were our own."

Russia: "You would do that for us, without charge?"

China: No, no. Right at the beginning, our army shouts 'Charge!'.