As some folks have pointed out it depends greatly upon the party as to what constraints you wish to place upon the contract or if you have one at all but let me wander done that path with a few musings of my own from past experience and maybe you will get some incites
As stated you have to tailor you adventure to the party, if you have a good aligned party that wants to be helpful then why would they keep the item for themselves as that is neither good nor helpful conversely if you part is evil, self-serving, or otherwise despicable in character (Chaotic Neutral to Chaotic Evil) then you need to deal with the party accordingly. So lets separate these two and look at them separately
Good Heroic Types
1) No Issue should be here with any contract they take to get any item
2) Just about any good person might higher them
3) The law might be willing to help them assuming it is not corrupt (aka evil)
4) Straight contracts in this case can be rather boring spice it up -- I liked the suggestion of sending
them to get a specific goose feather (or similar rather mundane seeming item) from a very
dangerous place and let them speculate on the why. For I have often changed what I was going
to do based off player speculation because as a player it is
FUN sometimes to correctly guess
what your GM is up to even if initially it was not what you as the GM were up to
Some of those other potential contracts work as well -- helping a PC's relatives, a PC's church,
PC's benefactor, sensai, mentor, etc....
In this vein I once sent the PCs to go find someone's nephew, but this was after they had proved they were
reliable by completing 3 previous missions, to make sure he was safe because the individual as their employer
had lost contact with their nephew. Upon completion of the task they would be paid for the information good
or bad. This got the players to the destination of their next slew of adventures and they almost forgot the reason
they had come there to begin with. Eventually they did find and ultimately rescue said nephew and got paid
for that mission but by then they were neck deep in adventures. Still the downtime was used for various tasks
while one of them took a ship back to their client to complete their contract and get paid.
Evil Group
1) First I want to say I never run non-heroic groups as I find no redeeming elements in such an endeavor
and player if they are truly playing their alignments are not going to be buddy buddy for all that long
so party doomed if played correctly
2) As someone mentioned a Thieves oriented adventure is what you are looking at here, and as such if
the employer is not prepared to handle betrayal then they deserve to get burned by the PCs conversely
burn me once and not only do you have assassins on your tail now but you will no longer get any more
missions from me or any of my allies. There is a reason why there is honor among thieves because
without no one trusts you and you cannot get any work unless it is to kill someone and if you succeed
at doing that you just might not get paid, burn me once, then I will burn you seems fair to me
3) Of course if you have a PCs church (aka deity) request them to do something for a reward and they
double-cross the deity well not only does that PC lose access to their clerical magic but they have a
pissed off evil deity now gunning for them.
4) Again I do not run this kind of characters also because eventually the whole concept of why most folks
play RPGs gets chucked out that window and that is being the hero of the story, there are very few
books/stories out there that are written from the villains point of view which should give you some
indication that it is not a very popular topic nor all that fun after a while especially if you are making
them deal with the true consequences of their actions. No one likes to live their life always looking
over their shoulder or worrying about the who is out gunning for them today.
Lastly I never force players to do anything as that by definition is railroading -- although if you want and the employer is evil you could have them force the players in some way shape or form but that should be only if the group is evil themselves as such this employer (good or evil) would have a way to make sure that the PCs are compliant with their end of the bargain (Suicide Squad for instance)