Jar pasta sauce resealed itself in fridge overnight. Is it safe to eat?
If you put hot (or even warm) sauce in a jar, seal it, and place the jar into the fridge, as it cools, it (as well as the air in the jar) will contract. The larger the temperature change, the greater the vacuum that will be created. 

Most likely, what you are seeing is warm sauce cooling and creating enough vacuum to suck in that button on the lid. If that's the case, it's perfectly safe to eat.

You say you opened the sauce, used some, and placed the rest (immediately??) into the fridge. That's a perfectly safe procedure for saving leftovers, so long as handling of the sauce did not introduce cross-contamination (ex via "double dipping").