We report the detection of the Mn-Kα line in the SN-IIb remnant, Cassiopeia A. Manganese (55Mn after decay of 55Co), a neutron-rich element, together with chromium (52Cr after decay of 52Fe), is mainly synthesized in core-collapse supernovae at the explosive incomplete Si-burning regime. Therefore, the Mn/Cr mass ratio with its neutron excess reflects the neutronization at the relevant burning layer during the explosion. Chandra's deep archival X-ray data of Cassiopeia A indicate a low Mn/Cr mass ratio with values in the range 0.10-0.66, which, when compared to one-dimensional SN explosion models, requires that the electron fraction be 0.4990 ≲ Y e ≲ 0.5 at the incomplete Si-burning layer. An explosion model assuming a solar-metallicity progenitor with a typical explosion energy (1 × 1051 erg) fails to reproduce such a high electron fraction. We can satisfy the observed Mn/Cr mass ratio if the explosive Si-burning regime was to extend into the O/Ne hydrostatic layer, which has a higher Y e. This would require an energetic (>2 × 1051 erg) and/or asymmetric explosion of a subsolar-metallicity progenitor (Z ≲ 0.5Z o˙) for Cassiopeia A. The low initial metallicity can be used to rule out a single-star progenitor, leaving the possibility of a binary progenitor with a compact companion. We discuss the detectability of X-rays from Bondi accretion onto such a compact companion around the explosion site. We also discuss other possible mass-loss scenarios for the progenitor system of Cassiopeia A.