  We investigated a cosmological model that allows a momentum transfer between
dark matter and dark energy. The interaction in the dark sector mainly affects
the behaviour of perturbations on small scales while the background evolution
matches the $w$CDM solution. As a result of the momentum transfer, these kinds
of models help alleviating the $\sigma_8$ discrepancy in the standard model,
but do not resolve the so-called $H_0$ tension. We confirm that this is indeed
the case by computing cosmological constraints. While our analysis tends to
favour $\sigma_8$ values lower than in $\Lambda$CDM, we do not find evidence
for a non-vanishing momentum transfer in the dark sector. Since upcoming galaxy
surveys will deliver information on scales and red-shift relevant for testing
models allowing momentum transfer in the dark sector, we also carried out
forecasts using different survey configurations. We assessed the relevance of
neglecting lensing convergence $\kappa$ when modelling the angular power
spectrum of number counts fluctuations $C_\ell^{\rm ij}(z,z')$. We found that
not including $\kappa$ in analyses leads to biased constraints ($\approx
1-5\,\sigma$) of cosmological parameters even when including information from
other experiments. Incorrectly modelling $C_\ell^{\rm ij}(z,z')$ might lead to
spurious detection of neutrino masses and exacerbate discrepancies in $H_0$ and
$\sigma_8$.
