I found the letters,they are quoted in the book
" The constants of nature from alpha to Omega " by John Barrow

The lady. mrs Ilse Roshental Schneider refers to the Max Plank's proposal of universal units , namely :

The gravitational constant : G
The light speed : c
The Max Plank constant : h

All other units can be derived from these 3 units
for example :

Time t = (Gh/c^5)^1/2 = 1.38 * 10^-43 sec

According to Plank , these are , so to speak, the foundation of the universe but Einstein disagrees
Einstein thinks that even these ones are somehow arbitrary units
The true "bricks" of the universe, the ones which even God could not change, must be dimensionless number same as the constants of math such as thenatural numbers or Pi or "e"
They must be , in other words, a ratio between omogenuo dimensional units
Einstein however admits that he his not be able to prove his claim