Let $M,N$ be manifolds with boundary. Let $f:\mathring{M}\to \mathring{N}$ be a diffeomorphism of their interiors. Does it extend to a diffeomorphism $M\to N$?
I suppose we can look at the problem locally, and then the problem is: if $U,V\subset \mathbb{H}^n$ are open sets in half-space, does a diffeomorphism $f:\mathring{U}\to \mathring{V}$ extend to a diffeomorphism $f:U\to V$?
EDIT: Seeing as the answer is negative, I now ask: what are sufficient conditions for the assertion to be true?
Take the inversion map on $(0, \infty)$ given by $f(x) = \frac{1}{x}$. This is differentiable with differentiable inverse but you cannot extend this to find $f(0)$