No can do w/ Matlab implementation; FFT and IFFT assume a fixed, constant sampling rate in the time domain which translates to a fixed df in the frequency domain.
To relate either to the actual sample rate and/or frequency requires knowing the sample rate and the FFT length going from time domain to frequency; and vice versa, that frequency resolution and length determines the transform back to time domain.
It's all just bookkeeping external entirely from the actual computation.
That's for the base form both directions; both functions do have the optional facility to interpolate/decimate the size of the result of the transform by a fixed number of elements in the output but this again presumes equi-spaced, not arbitrary time/frequency.
Whether there's a generalized form in Matlab at the File Exchange or somewhere else on the web I've not researched.
