  The effects of pressure on a quantum spin liquid are investigated in an
organic Mott insulator $\kappa$-(ET)$_2$Ag$_2$(CN)$_3$ with a spin-1/2
triangular lattice. The application of negative chemical pressure to
$\kappa$-(ET)$_2$Cu$_2$(CN)$_3$, which is a well-known sister Mott insulator,
allows for extensive tuning of antiferromagnetic exchange coupling, with
$J/k_{\rm B} = 175 - 310$ K, under hydrostatic pressure. Based on $^{13}$C
nuclear magnetic resonance measurements under pressure, we uncover universal
scaling in the static and dynamic spin susceptibilities down to low
temperatures $\sim 0.1k_{\rm B}T/J$. The persistent fluctuations and residual
specific heat coefficient are consistent with the presence of gapless low-lying
excitations. Our results thus demonstrate fundamental finite-temperature
properties of quantum spin liquid in a wide parameter range.
