[en] In this work, we are interested in the breaking of the isospin symmetry in the N ≅ Z nuclei and in its effect on the matrix element of super-allowed 0⁺ → 0⁺ Fermi β transitions in the case of the β⁺ decay of the ⁵⁰Mn. Within the framework of the Highly Truncated Diagonalization microscopic Approach (HTDA), dedicated to the description of correlations beyond the mean field and conserving explicitly the particle number, we have studied (in particular) the role played by pairing correlations in the breaking mechanisms of this symmetry in the ground state of N ≅ Z nuclei. A sensitivity study of the isospin mixing, as a function of the strength of the residual interaction describing the pairing correlations in HTDA, has been carried out and an interpretation of the mechanisms at work has been proposed in terms of an approximation developed in this work. This study has pointed out the complexity of a good treatment of the isospin symmetry, in the description of the breaking sources as well as in the reduction of model biases. We have also paid attention to the necessity of a very fine description of the correlated wave functions in such a problem. More precisely, we have obtained a value for the isospin mixing correction δ_C to the Fermi transition matrix element of (0.2 ± 0.1)%. This value has been compared to those obtained in other approaches. Taking account of the neglected effects in our work, our value of δ_C is expected to be a lower bound. (author)