Optical solitons in a medium with both resonant and non-resonant (Kerr-type) nonlinearities have earlier been shown to exist only under a definite relationship between the parameters of medium and radiation. In this paper, we study the influence of a small deviation from this relationship on the distortion of the soliton propagation. Our approach is realized via an algebraic version of the soliton perturbation theory formulated by means of the Riemann boundary problem. Corrections for the soliton parameters are calculated and an expression is obtained which describes the soliton shape deformation. Conditions of the soliton tail appearance are also analysed.