1) He was given a bottle of the magic potion, to use in cases of extreme emergency.
2) He was given a bottle of the magic potion, to be used in cases of extreme emergency.

The comma could indicate an afterthought.
Could it also be used if one wanted to postmodify 'the magic potion' non-restrictively?

In other words, could the sentences be the equivalents of:
a) He was given a bottle of the magic potion, which is to be used in cases of extreme emergency.


Here we have a general rule: the magic potion is to be used in cases of extreme emergency.

Gratefully,
Navi.

HAPPY NEW YEAR!