  The static properties of the heaviest residue and unbound particles produced
in central $^{64,58}Ni$ on $^{64,58}Ni$ collisions at 50 MeV/nucleon are
predicted within the BUU transport model, in order to explore the sensitivity
of those observables to the density dependence of the symmetry energy. We
include fluctuations in the collision integral and use a meta-modelling for the
mean-field which allows an independent variation of the different empirical
parameters of the equation of state. We find that the isospin ratio of
pre-equilibrium particles is a good estimator of the stiffness of the symmetry
energy, in agreement with previous works. In addition to that, whatever be the
functional form of the equation of state, we show that a higher symmetry energy
at subsaturation densities leads to an increased size and isotopic ratio for
the heaviest residue. This is understood in terms of energy sharing between the
pre-equilibrium particles and the (quasi)fused system. The combination of the
two observables might be an interesting tool to constrain the different density
dependence below and above saturation, which is linked to the relatively poorly
known parameter $K_{sym}$.
