So, one of my Acolytes is considering taking this package for a new game.
I was wondering; would he suffer the Corruption point penalty, the Mask of Seaming and the Counting the Cost problems; or does he just get the benefit of having the part of the pact?

I guess it's up to you to decide. I'd say he only starts with this beneficial side of the Pact, but with time it may turn out that he is more and more affected by whatever power he deals with. Of course Corruption points should apply normally.

Mm, fair enough. I may penalize him only half of the Corruption points, and then extra ones later. Was just wondering if there was an official ruling on that jazz or not.

What is this package you are talking about? It sounds like a buff that allows new characters to have all there stuff from old characters, if this is true, what book is it in, and how dies it work?

By RAW he gets the benefits of one of the pact effects and none of the disadvantages associated with a proper Dark Pact.

I like my idea better.

He wanted an answer (presumably RAW), I gave him one.

Keep in mind the character has not actually entered into a pact with a daemon. He just knows enough of the daemon's nature to use it for his own ends. He or she specifically gains the effects of one boon, and the ability to invoke the entity into existence as per a Dark Pact.

This is itself not a Dark Pact, RAW-wise. That might not stop your GM from writing some backlash into abusing a Daemon's powers if it becomes excessive, and you could quite rightly expect the daemon itself to try and retaliate (when possible), or some of its other followers from elsewhere try and liberate it from the commanding PC in question.

You may do something like this: Daemon let the character know part of its name, but with time it may turn out that the Daemon actually has its own sceme and such an Adept only helps him fulfill it. (It's somehow similar to Eisenhorn's relation with Cherubael.)