  Applying pressure around megabar is indispensable in the synthesis of
high-temperature superconducting hydrides, such as SH$_3$ and LaH$_{10}$.
Stabilizing the high-pressure phase of hydride around ambient condition is a
severe challenge. Based on the density-functional theory calculations, we give
the first example that the structure of hydride CaBH$_5$ predicted above 280
GPa, can maintain its dynamical stability with pressure down to 1 GPa, by
modulating the charge transfer from metal atoms to hydrogen atoms via the
replacement of Ca with alkali metal atoms e.g. Cs, in which the [BH$_5$]$^{2-}$
anion shrinks along $c$ axis and expands in the $ab$ plane, experiencing an
anisotropic virtual high pressure. This mechanism, namely charge transfer
modulated virtual high pressure effect, plays a vital role in enhancing the
structural stability and leading to the reemergence of
ambient-pressure-forbidden [BH$_5$]$^{2-}$ anion around 1 GPa in CsBH$_5$.
Moreover, we find that CsBH$_5$ is a strongly coupled superconductor, with
transition temperature as high as 98 K, well above the liquid-nitrogen
temperature. Our findings provide a novel mechanism to reduce the critical
pressure required by hydrogen-rich compound without changing its crystal
structure, and also shed light on searching ambient-pressure high-temperature
superconductivity in metal borohydrides.
