Ultra posse nemo obligatur - "No one is obligated (to do) more than he can." Specifies that one should do what he can to support the community, but since everyone has different levels of ability, it cannot be expected that all will perform the same.⏎
Solve et repete - Respect your obligation first, then you can ask for reimbursement. Used in those situations in which one of the two (or more) parties needs to complete his obligation before being allowed to ask for the opposite obligation to be respected by his counter party. Usually this principle is used in fields and subjects in which a certain general steadiness or uniformity of the system has been considered a relevant value by the legislator. The case is typical of service contracts with repeated obligations (like with gas, water, electricity providers and similars), in which irregularities on one side cannot be balanced if not in a regular situation (i.e., of payments) on the other side. The customer, for example, might be asked to pay regularly the new bill, before contesting the previous one in which he found irregular calculations, and asking for a balancement with newer bills; he thus cannot by himself determine a discount in the next payment.⏎
Vigilantibus non dormientibus aequitas subvenit - "Equity aids the vigilant, not the sleeping." Concept that if an opposing party unreasonably delays bringing an action, that it is no longer considered just to hear their claim, due to fundamental changes in circumstance brought upon by their delay.⏎
Res inter alios acta vel iudicata, aliis nec nocet nec prodocet - What has been agreed/decided between people (a specific group) can neither benefit nor harm a third party (meaning: two or more people cannot agree amongst each other to establish an obligation for a third party who was not involved in the negotiation; furthermore, any benefit that may be established will have to be accepted by the third party before it can be implemented).⏎
Rex non potest peccare - "The king can do no wrong." Used to describe the basis for sovereign immunity.⏎