The mechanics of how magic work have largely been left up to the DM.
In general, I've always interpreted summoning spells as conjuring a spirit version of the creature from the outer planes of some sort. As a spirit, it can't die by normal means - only be banished back to the place where it comes from.
As DM, I exercise the right to treat conjured beings as NPCs. This has _never_ come up in the game but would come up if the PC's conjured a being and then tried to get it to violate its basic nature - say forcing a celestial to commit an act of great evil, or forcing an infernal to serve the cause of good in an overt way. A being conjured to perform an act in gross violation of its nature simply refuses to act.
Part of the reason that this then never comes up, as it tends to force players ahead of time to think about what they want to accomplish before choosing what to summon. This involves then almost no limitation on how the players use the spell, and at the same time preserves my sanity as a DM regarding how the spell works (because forcing a fiend to cooperate is well beyond the ability of a low level spell). Conjured beings show up because they want to, and are getting something out of it (either the opportunity to advance a cause or just the opportunity to commit acts of violence), and not because the spell itself is powerful enough to force them to do so.
I'd be quite willing to have the beings in question be repeat 'customers', but I would not allow you to pre-stocking conjured entities with magical items or gifts. Conjured beings can't take anything with them when they go back to the spirit worlds (otherwise, you could command them to grab someone, and then dismiss them to permanently planeshift away a villain).