Would it be all right if we left a little early?
Would it be all right if we leave a little early?
What's the difference between the two sentences?
The only difference is 'left' vs. 'leave'.
This is a difference of "first" or "second" conditional. The first conditional (with present tense) is used for hypothetical events, the second conditional is also for hypothetical events, that the speaker believes are unlikely.
In this case, the natural conditional to use would be "if we leave". However using "if we left" may be used because it is more tentative. It isn't a literal second conditional suggesting that the event is unlikely. It is used to tentatively inquire about leaving early.
So in practice the only difference is that the second is marginally more polite than the first.