Transverse slice emittance is one of the most important properties of high-brightness electron beams for free-electron lasers (FELs). The photo injector test facility at DESY in Zeuthen (PITZ) develops high-brightness electron sources for modern FELs. With a 23 MeV, 1 nC beam at PITZ the experimental slice emittance characterization with the quadrupole scan technique is complicated by strong space charge effects. Combining the slit scan technique with a transverse deflecting cavity (TDS) allows for time-resolved emittance measurements of such a space-charge-dominated beam. The first proof-of-principle results of slice emittance measurements at PITZ based on the'TDS + slit scan'-technique are presented in this paper.