Difference form Time to Frequency domain and from Frequency to Time domain.

Hello there, I am trying to understand why I am getting a fraction difference after converting to frequency domain and back to time domain. The first question is how can i get the exact time signal values after transformation? Another question is why if I modified one frequency bin and transform back to time domain, the output vector is complex numbers?

    x = floor((100-1).*rand(16,1) + 10);    % Generate 16 random integer
    y = fft(x);                             %FFT transform
    x2=ifft(y);                             %back to Time Domain
    diff = x-x2;                            %Small fractions at variable points