While doing research for another recent question, I ran into two back-to-back lines that seem to contradict each other. From the PFSRD, emphasis mine:
Each morning, when a spellcaster prepares spells or regains spell slots, he can also imbue one staff with a portion of his power so long as one or more of the spells cast by the staff is on his spell list and he is capable of casting at least one of the spells. Imbuing a staff with this power restores one charge to the staff, but the caster must forgo one prepared spell or spell slot of a level equal to the highest-level spell cast by the staff.
Say you have a simple fire staff with burning hands, fireball, and maybe even delayed blast fireball. The first line says that any spellcaster capable of casting any 1 of the 3 should be able to recharge that staff. But the second line states that the recharging spellcaster needs to give up a slot of the highest spell level the staff holds specifically. So how does one reconcile the two statements?
I have a few ideas… in order from least to most forgiving to the player ^_^
a) The spellcaster cannot recharge the staff (statement 2 being more specific overrides statement 1).
b) The spellcaster can recharge the staff by forgoing his highest available spell slot (gets as close as possible to fulfilling statement 2 while being allowed by statement 1).
c) The spellcaster can recharge the staff by forgoing his highest spell slot that matches a spell the staff can cast (so in the example, a wizard without fireball could forgo a first-level slot for burning hands, but a wizard with 3rd to 6th level spells would have to give up a 3rd level instead; the most literal combination of the two statements).
d) The spellcaster can recharge the staff for free (statement 1 clearly says "he can recharge this staff" but the cost defined in statement 2 is unpayable; ergo the cost is waived but the explicitly allowed effect is still granted).
Best Answer
The text is clear: the spellcaster must a) be able to cast at least one of the spells in the staff and b) possess a slot equal to or greater than the highest-level slot. Most of the time, this means they must be able to cast any of the (one or more) highest-level spells in the staff.
Occasionally, a multiclass caster might be able to satisfy both conditions with casting from different classes. A Wizard 1/Cleric 13, for example, can of course give up a (Cleric) 7th-level slot for DBF, and can* cast burning hands: boom, mission accomplished, staff can be recharged.
(For clarity, this is most similar to what's written as interpretation 2, except that there is no requirement for the character's highest-level slot, only a slot that matches the staff's highest-level spell. A Wizard 17 with such a staff would not need to sacrifice one of their precious 9ths.)
*With a suitable entry in their spellbook, anyway