Mathmatic question

3 men went into a bookstore. they bought a book by prise 3000 dollors, each of them paid 1000 dollors. (1000+1000+1000=3000)

After they went out the sellor understood his mistake, the real prise was 2500 but he said 3000 so he asked his co worker to get back 500 dollors to those guys. He accepted to do it.(3000-2500=500)

The co-worker followed them until he found them, he kept 200 dollors for his own and then paid 100 dollors to each of them. (200+100+100+100=500)

Now the question is here each of them paid 900 so all of them paid 2700 for the book and 200 for co-worker so 2700+200=2900, where is the 100 dollors??

tell me your answers!