You do not clear about the unit of side, I'm considering it as mm. If it's different just change the unit.
Solution:
Side length of cube (a) = 500×10^-3 mm = 500/1000 = 0.5mm
We know Volume of cube (V) = a^3
Volume of grain of salt = (0.5)^3
= 0.125 = 1.25×10^-2 mm^3