Brine shrimp is not a healthy diet, it is a "treat."
It is generally held that brine shrimp is the nutritional equivalent of Twinkies. You could eat Twinkies for every meal, and you might stay alive and look OK...however, after several weeks of this diet, you would begin to suffer from malnutrition, even though you would be getting fat. Eventually, your body would start to break down and you would end up very sick.
Feeding your mandarin a straight diet of brine shrimp would lead to the same results in your mandarin...he might even look fat, but he will be malnourished.
Just so you know, you are doing it again...
You obviously want to keep him, and you are searching for someone to tell you it is OK to keep him. Every post on this board has given you the advice to take him back, and I guarantee you that will continue to be the advice here. The advice from experienced aquarists and "words from the wise" are to take it back. Ultimately, it will be your decision to heed or ignore the advice, but don't ask for help and then argue with the advice. The people here are responsible aquarists, and are just not going to tell you what you want to hear, they are going to tell you what is in the best interests of your fish.