Learn Hardcore Serbian: Polazak je u osam, a povratak u sedam. - The departure is at eight, and the return at seven.

So the sentence uses nouns, not verbs like we leave / we return. Serbian often uses this kind of noun-based phrasing in schedules, announcements, and travel contexts.

Why is there je in the first part, but not in the second?

Because the second je is simply omitted as understood.

The full version would be:

Polazak je u osam, a povratak je u sedam.

In Serbian, when two clauses are parallel, the repeated verb is often left out in the second one to avoid sounding repetitive. English does this too sometimes:

Departure is at eight, and return at seven.

So the omission is natural and normal here.

Why is a used here instead of i or ali?

A often links two ideas by contrasting or balancing them.

Here it works like:

while
and as for
whereas

So Polazak je u osam, a povratak u sedam feels like:

departure is at eight, while return is at seven