According to band structure calculations, the Fermi surface of the quasi-two dimensional metal θ-(ET)4ZnBr4(C6H4Cl2) illustrates the linear chain of coupled orbits model. Accordingly, de Haas-van Alphen oscillations spectra recorded in pulsed magnetic field of up to 55 T evidence many Fourier components, the frequency of which are linear combinations of the frequencies relevant to the closed α and the magnetic breakdown β orbits. The field and temperature dependence of their amplitude are quantitatively accounted for by analytic calculations including, beyond the Lifshitz-Kosevich formula, second-order terms in damping factors due to the oscillation of the chemical potential as the magnetic field varies. Whereas these second-order terms are negligible for the orbits α, β and 2β-α, they are solely responsible for the 'forbidden orbit' β-α and its harmonic and have a significant influence on Fourier components such as 2α and β+α, yielding strongly non-Lifshitz-Kosevich behaviour in the latter case.