We report, for the first time, the detection of the Mn-K$alpha$ line in the Type IIb supernova (SN IIb) remnant, Cassiopeia A. Manganese ($^{55}$Mn after decay of $^{55}$Co), a neutron-rich element, together with chromium ($^{52}$Cr after decay of $^{52}$Fe), is mainly synthesized 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. Chandras 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 $lesssim Y_{rm e} lesssim$ 0.5 at the incomplete Si burning layer. An explosion model assuming a solar-metallicity progenitor with a typical explosion energy ($1 times 10^{51}$ erg) fails to reproduce such a high electron fraction. In such models, the explosive Si-burning regime extends only to the Si/O layer established during the progenitors hydrostatic evolution; the $Y_e$ in the Si/O layer is lower than the value required by our observational constraints. We can satisfy the observed Mn/Cr mass ratio if the explosive Si-burning regime were to extend into the O/Ne hydrostatic layer, which has a higher $Y_{rm e}$. This would require an energetic ($> 2 times 10^{51}$ erg) and/or asymmetric explosion of a sub-solar metallicity progenitor ($Z lesssim 0.5Z_{odot}$) 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 (white dwarf, neutron star or black hole). 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.
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