Let $X$ and $Y$ be arbitrary sets and $f:X\rightarrow Y$ an isomorphism. Prove that there exist a transformation $g:Y\rightarrow X$ such that $f\circ g$ is the identity in $Y$.
As X and Y havn't a structure to preserve, $f$ is just a bijective function; hence exist a function $g$ such that $f\circ g$ is the identity in Y. In particular this function is a transformation.