The answer you are getting
xByte[0] :-127
xByte[1] :-82
is right.
This is called 2's compliment Represantation. 1st bit is used as signed bit.
0 for +ve
1 for -ve
if 1st bit is 0 than it calculates as regular. but if 1st bit is 1 than it deduct the values of 7 bit from 128 and what ever the answer is presented in -ve form.
In your case
1st value is10000001
so 1(1st bit) for -ve and 128 - 1(last seven bits) = 127
so value is -127
For more detail read 2's complement representation.