Learn Hardcore Indonesian: Saya minta kembalian kepada kasir.

Formal/neutral: Saya minta kembalian kepada kasir.
Neutral/informal: Saya minta kembalian ke kasir.
Colloquial: Saya minta kembalian sama kasir. In everyday speech, ke or sama sounds more natural. Kepada feels formal or written.

Does this sentence sound like something you would actually say at a counter?

It’s grammatical but sounds like you’re reporting the action to someone else. At the counter you’d usually use a short, soft request such as:

Permisi, kembaliannya belum.
Boleh minta kembaliannya?
Mbak/Mas, kembaliannya ya. Those are more idiomatic than the full sentence with kepada kasir.

Is minta too casual? What about meminta or mohon?

minta = everyday neutral and perfectly fine in speech.
meminta = more formal/politer, common in writing or careful speech.
mohon = very formal “to request/plead,” not natural for asking a cashier for change. To sound polite, add softeners: Boleh minta kembaliannya?, Tolong, saya minta kembaliannya.

Should it be kembaliannya (with -nya) instead of kembalian?

Use -nya when you mean “the (specific/my) change,” which is very common at the counter:

Saya minta kembaliannya. = I’d like my change (definite).
Saya minta kembalian. = I’m asking for change (in general; less specific). You can also say kembalian saya to make “my” explicit.

Do I need to say uang kembalian, or is kembalian enough?

kembalian already implies money, so it’s usually enough. uang kembalian is not wrong and some people do say it, but it’s a bit redundant. Examples:

Mbak, kembalian saya belum.
Mbak, uang kembaliannya belum. (also heard, just wordier)

How do I show tense (past/future) since minta has no tense?

Add time/aspect markers:

Past: Tadi saya minta kembalian ke kasir. / Saya sudah minta kembalian ke kasir.
Future: Saya akan minta kembalian ke kasir.
Progressive: Saya sedang minta kembalian ke kasir.

Is this word order okay: Saya minta kepada kasir kembalian?

That word order is unnatural. Keep the thing requested right after the verb, then the recipient:

Natural: Saya minta kembalian kepada/sama kasir. A very formal pattern is meminta kepada [orang] agar…, but that introduces a clause, not a noun like kembalian.

What’s the difference between kepada kasir and dari kasir?

kepada kasir highlights whom you direct the request to (the addressee).
dari kasir highlights the source you expect to receive it from. Both can occur, but with minta, kepada (or colloquial ke/sama) is the default:
Saya minta kembalian kepada/ke/sama kasir.
Saya minta kembalian dari kasir. (possible, but focuses on the source)

How would I politely address the cashier?

Use common address terms instead of saying kasir to their face:

Female (younger/adult): Mbak
Male (younger/adult): Mas
Older female: Bu
Older male: Pak Examples: Mbak, boleh minta kembaliannya? / Pak, kembaliannya ya.

When should I use saya, aku, or gue/gua here?

saya: polite/neutral, safe with strangers and service staff.
aku: informal with friends/family.
gue/gua: Jakarta-style colloquial; use only if appropriate for the social context. All fit grammatically: Saya/Aku/Gue minta kembaliannya.

Is minta kembalian the same as asking to break a large bill?

Not exactly. minta kembalian = asking for change that’s owed after paying. To break a bill, say:

Bisa tukar uang seratus ribu jadi pecahan kecil?
Ada uang kecil?
Bisa minta pecahan kecil?

How do I say “keep the change” in Indonesian?

Tipping isn’t standard everywhere, but if you want to let them keep it:

Tidak usah kembaliannya.
Kembaliannya untuk kamu ya. (very clear; use cautiously and politely)
Kembaliannya ambil saja. In many contexts, people will refuse once or twice; this is normal politeness.

What’s the difference between kembalian and pengembalian?

kembalian = the small change you get after paying. Example: Kembaliannya lima ribu.
pengembalian = a return/refund/act of returning. Examples: Pengembalian dana, pengembalian barang.

How do I say it in the passive or with “give”?

Examples:

Active with “give”: Kasir memberi saya kembalian.
Passive: Saya diberi kembalian oleh kasir. / Kembaliannya diberikan kasir ke saya.
Focus on the change: Kembaliannya sudah dikasih belum?

Can I omit kepada kasir or even saya?

Yes, if context makes it clear:

(Saya) minta kembaliannya. (drop saya in casual speech)
If you’re talking to someone else and need to specify: Saya mau minta kembalian ke kasir.

Any common mistakes to avoid with this sentence?

Writing ke pada (two words). Correct: kepada.
Overusing mohon for everyday requests at a shop.
Confusing kembalian (small change) with pengembalian (refund).
Sounding too blunt: avoid things like Kasih kembalian!; soften with permisi, boleh, ya, tolong.