  Galaxy-galaxy or galaxy-quasar lensing can provide important information on
the mass distribution in the Universe. It consists of correlating the lensing
signal (either shear or magnification) of a background galaxy/quasar sample
with the number density of a foreground galaxy sample. However, the foreground
galaxy density is inevitably altered by the magnification bias due to the mass
between the foreground and the observer, leading to a correction to the
observed galaxy-lensing signal. The aim of this paper is to quantify this
correction. The single most important determining factor is the foreground
redshift z: the correction is small if the foreground galaxies are at low
redshifts but can become non-negligible for sufficiently high redshifts. For
instance, we find that for the multipole l=1000, the correction is above
1%*(5s-2)/b for z<0.37, and above 5%*(5s-2)/b for z<0.67, where s is the number
count slope of the foreground sample, and b its galaxy bias. These
considerations are particularly important for geometrical measures, such as the
Jain and Taylor ratio or its generalization by Zhang et al. Assuming
(5s-2)/b=1, we find that the foreground redshift should be limited to z<0.45 in
order to avoid biasing the inferred dark energy equation of state w by more
than 5%, and that even for a low foreground redshift (< 0.45), the background
samples must be well separated from the foreground to avoid incurring a bias of
similar magnitude. Lastly, we briefly comment on the possibility of obtaining
these geometrical measures without using galaxy shapes, using instead
magnification bias itself.
