I'm not very good at math, but I personally think that Xiaoming only paid 99.9 yuan after receiving 99.8 yuan, which means that Xiaoming earned 0. 1 yuan every time, which means that he earned 99.8 ÷ 2 = 44.9 yuan and paid 99.9 -44.9 yuan = 50 yuan for the last time.

So I think Xiaoming finally lost 50 yuan.