Quadrature subsampling result not as expected
Your calculation is correct; sif has a harmonic at 308 kHz. Given the way you have set up the problem, you can't get that exact value using the FFT because you don't have a frequency bin at that frequency. According to my calculations, the nearest bin is at 305 kHz, and I do see a peak at that frequency. Since there is no bin at the signal's frequency, the energy will leak: bins in the neighborhood of the peak will also get some of the energy of the 308 kHz signal.

Leakage is very hard to avoid. Assuming a fixed sampling frequency, you can calculate the number of samples you need to have a bin at exactly 308 kHz. However, it's very likely that you will then have a non-integer number of cycles of sif, so you will have leakage again. This effect can be reduced by using windowing.

There are many related questions in this website (for example, this one). Search for 'fft bin leakage' to find many more.