  We present high-field magnetization and $^{79}$Br nuclear quadrupole
resonance (NQR) and $^{125}$Te nuclear magnetic resonance (NMR) studies in the
weakly coupled Cu$^{2+}$ ($S=1/2$) tetrahedral system Cu$_2$Te$_2$O$_5$Br$_2$.
The field-induced level crossing effects were observed by the magnetization
measurements in a long-ranged magnetically ordered state which was confirmed by
a strong divergence of the spin-lattice relaxation rate 1/T1 at T0=13.5 K. In
the paramagnetic state, 1/T1 reveals an effective singlet-triplet spin gap much
larger than that observed by static bulk measurements. Our results imply that
the inter- and the intra-tetrahedral interactions compete, but at the same time
they cooperate strengthening effectively the local intratetrahedral exchange
couplings. We discuss that the unusual feature originates from the frustrated
intertetrahedral interactions.
