No, it's not correct. I'd like to correct it for you, but I'm not 100% sure what it's trying to say. You can leave out the word "today" and it makes sense. (''I used to finish my work before night).''
"I used to" means you did something in the past. (I used to smoke, I used to eat too much).
If you want to say you finished work before night arrived in the past, you can say, "I used to finish working in the late afternoon before darkness."