A possible alternative would be to have planar binding spells have an alternate option where you make a long term contract in exchange for some favor (requesting some important item offered in tribute or sacrificing xp or even full levels) or obligation (xp penalty, persistent stat penalties etc for the duration of the agreement). (this is how they work in P&P, there's a 1 round/ level option where you just call them for temporary service and a random planar answers and helps, OR you can negotiate a longer contract that can last for weeks or months but will only aid you in causes they're interested in with a specific individual).
Which if negotiations are successful they replace your familiar with that creature for the duration of the agreement. (clerics who successful negotiate a deal are bestowed summon familiar for the duration of the agreement). If a planar ally dies, they cannot be re-summoned for at least 1 full in-game day from their time of death, but otherwise suffer no consequences.
(a druid with elemental shape can choose to permanently replace their animal companion with an elemental at least 2 categories lower than their elemental shape-form. This isn't a contract and costs nothing to do but they can't change their elemental ally for at least 1 week and they cannot be resummoned for at least 1 full in-game day if they die).
Attempting to use a Good planar being for obviously evil actions will immediately break the contract (basically anything that could trigger a paladin falling). Evil planar beings don't care what they're used for (though having a contract with an evil planar being will cause your alignment to drift slowly towards evil by 1 point per in-game week).
Additionally evil planar beings typically have "easier" negotiations but tend to cause their owner's more grief (they're more likely to demand xp penalties, xp loss or loss of levels or alternatively, you could offer them a slain person's body as a sacrifice (counts as destroying the remains) and results in a large chunk of evil points, with the amount of bargaining power depending on the level of the person), while Good planars are harder to negotiate with (generally requiring you make an offering of some sort about some great evils you've slain or opposed but might also take monetary distributions that will be sent to help goodly causes) but generally won't penalize their contract-maker too much (they might impose a stat/skill penalty as a burden to bare in exchange for their service if you don't have any else to offer).
influence, bluff, antagonize, appraise, lore can all be used to attempt to haggle the deal to get more time than than normal or at a lesser cost than normal for the same time. However they can also result in much higher costs if you do poorly. (Once you've made contact with a planar being to negotiate with, you are locked to that planar being for 1 week. After which any haggling results are reset as you're assumed to have contacted a new planar being). You cannot make a new contract until your current one expires or is voluntarily ended (re-casting contact planar being while under contract allows you to end your contract/pact). (when negotiating with good planars, a character closer to their alignment has better starting negotiation on price. Evil planars offer significantly better deals for characters opposed to them, but making a deal with any evil planar immediately results in 5 points towards evil + 1 per week (paided at the start of the week. This means the cost of a 1 week contract is 6 evil points even if you cancel it before the end of the week). Characters who make a contract with a good planar gain +5 good (even evil characters can potentially make a successful contract but it's extremely difficult as they essentially have to "prove" they're repenting of their actions for it to be allowed), but gain +10 evil if the contract breaks early due to evil actions.
alternatively, you could make a pact, which grants you a semi-permanent benefit to some stat or skill instead, with the ability to call your planar patron for aid in battle once per week without the need of using a spell (this works as per the spell used to make the pact but as if affected by the extend spell meta-magic), that lasts for the duration of the pact. (again, same deal as good planar vs evil planar. If you have a good pact, doing any evil actions that would cause a paladin to fall will end the pact, while having an evil pact will slowly twist you towards evil). This could be up to +4 in 1 stat or +2 in two stats Or +3-5 (most skills are in groups of 3 with +5 but a few have 4 which only get +3) in several related skills or a +10 to one. (the exact magnitude will depend on your bargaining power relative to the time of the pact you're trying to negotiate with some options not even available if your bargaining score isn't high enough for the duration you've got in mind).
