I could see it a couple of ways.
1) You're using a potion that someone gives you. That's specifically allowed. Keeping it for later? Not so much.
2) You're "borrowing" the potion for immediate use, thus incurring a debt that has to be repaid, and essentially making yourself poorer. That seems okay as well, so long as you actually try to repay the debt.
In neither case could/should you be allowed to keep the potion for later.
Taking a different tack, I could see someone asking you to keep a potion in reserve, to be used on them if they go down. So long as those were the terms, making it clear that this was not your potion to use as you would but rather that you were accepting an obligation to help the potion's owner, I'd see that as being okay as well.
If you took that potion yourself, in an emergency situation, it would be stealing, a decidedly un-Lawful act. Bad thing for a Monk to do.
But then, this is just my opinion, and a poorly informed one at that.