According to the meaning of the question, the money for the first book sale is 100+40 = 140 yuan. Since the unit price of book sales is 2.8 yuan, the first purchase quantity is 140/2.8 = 50, so the unit price of each book is 100/50 = 2 yuan.

Because each book increases the second purchase cost of 0.5 yuan, the unit price of each book is 2.5 yuan.

The owner spent 150 yuan to buy it, so the number of books is 150/2.5=60.

But after 80% of the books are sold out, they are all sold at 50% off, that is, at half price, so the books sold at the original price are 60*80%=48, so the books sold at half price are 12.

The total cash purchased is 48 * 2.8+12 * 50% =151.2 yuan.

Excluding the purchase money is the profit he earned, and the cut profit is151.2-150 =1.2 yuan.