Cabe aos condôminos aprovar a contribuição extraordinária.

Caber does often mean to fit:
Não cabe na mala = It doesn’t fit in the suitcase.

But it also has a more formal meaning: to be up to someone, to fall to someone, or to be someone’s responsibility/role.

So in this sentence, Cabe aos condôminos... means something like It is up to the condo owners... or It falls to the condo owners...

Because aos condôminos is not the grammatical subject here.

In this structure, aos condôminos means to the condo owners / for the condo owners, and the action aprovar a contribuição extraordinária is what functions as the subject idea.

That whole action is treated as a single thing, so the verb stays singular: cabe.

A helpful comparison:

• Cabe aos condôminos aprovar a contribuição extraordinária.
• Essas decisões cabem aos condôminos.

In the second sentence, essas decisões is a plural subject, so cabem is used.

Because this meaning of caber normally uses the preposition a:

• caber a alguém = to be up to someone

So:

• a + os = aos
• aos condôminos = to the condo owners

You can think of it as part of the pattern:

• Cabe ao síndico decidir isso.
• Cabe aos condôminos aprovar isso.

The most standard pattern for learners is:

caber a alguém + infinitive

So:

• Cabe aos condôminos aprovar...

That is the safest and most neutral choice.

You may sometimes come across a personal infinitive in very formal writing, but in this structure aprovar is the normal form to learn and use first.

Also, English speakers sometimes want to use a finite clause such as que aprovem, but that would require a different sentence structure.

In Brazilian Portuguese, condômino usually means a person who owns a unit in a condominium or is legally one of the co-owners in that property arrangement.

So it is more specific than morador:

• morador = resident
• condômino = condo owner / co-owner

A tenant may be a morador, but not necessarily a condômino.

In condo/building management language, contribuição extraordinária is an extra charge or special assessment beyond the regular monthly condo fee.

It is usually collected for things like:

• major repairs
• renovations
• emergency expenses
• unexpected building costs

So it is not the normal recurring fee, but an additional one.

Yes. It sounds formal, administrative, and somewhat legal.

You might see this kind of wording in:

• condo bylaws
• meeting minutes
• official notices
• legal or administrative documents

In more everyday speech, people might say something like:

• Os condôminos precisam aprovar a contribuição extraordinária.
• Quem tem que aprovar a contribuição extraordinária são os condôminos.

Yes. Portuguese allows some flexibility here.

These are all possible, with slightly different emphasis:

• Cabe aos condôminos aprovar a contribuição extraordinária.
• Aos condôminos cabe aprovar a contribuição extraordinária.
• A aprovação da contribuição extraordinária cabe aos condôminos.

The version with Aos condôminos cabe... sounds especially formal and slightly more emphatic.

Using a usually suggests a specific extraordinary fee already known in the context, such as one being discussed in a meeting or mentioned earlier.

So:

• a contribuição extraordinária = the extraordinary assessment / that specific extra fee
• uma contribuição extraordinária = an extraordinary assessment / some extra fee

Portuguese also uses definite articles more often than English, so a can sound natural even where English might not always use the.