Learn Hardcore Indonesian: Misalnya gaji belum masuk, kami menunda belanja besar. - For example, if the salary hasn’t come in, we postpone big shopping.

Misalnya means for example. Here it introduces one example condition under which the action happens.
It is not itself the conditional marker; the conditional idea is implied from context. You can imagine a fuller version: Misalnya, kalau gaji belum masuk, kami menunda belanja besar.
A comma after misalnya is stylistically preferable: Misalnya, gaji belum masuk, ... The sentence is still understandable without it.

Is it okay to omit an explicit “if” (kalau/jika) like this?

Yes. Indonesian often implies a condition without an explicit marker when the context (or a word like misalnya) makes the relationship clear. If you want it explicit, say:

Kalau/Jika gaji belum masuk, kami menunda belanja besar.

Why is it belum and not tidak?

Belum = “not yet,” with an expectation it will happen later. Gaji belum masuk suggests the salary is expected to arrive.
Tidak = plain “not,” with no expectation. Gaji tidak masuk would mean “the salary doesn’t come in” (as a rule) or “didn’t come in” (this time) without the built‑in “yet” nuance.
You can emphasize the ongoing wait with masih belum: Gaji masih belum masuk.

What does masuk mean with money? Does it need ke rekening?

Masuk literally “enter,” but with money it means “to be credited/come in.” Gaji (sudah) masuk = “the salary has been credited.”
You may add the destination: Gaji belum masuk ke rekening (saya/kami). It’s often omitted because it’s understood.

Are there alternatives to masuk here, and are there nuances?

Yes:

cair (funds disbursed/cleared): Gaji belum cair.
ditransfer (transferred): Gaji belum ditransfer.
dibayar/dibayarkan (paid): Gaji belum dibayar.
diterima (received): Gaji belum diterima. All are natural; masuk and cair are very common in everyday talk about pay day.

Why is it kami and not kita?

Kami = we (excluding the listener).
Kita = we (including the listener). If you’re talking to someone who is part of the group doing the postponing, use kita. Otherwise, kami is right.

Why is it menunda belanja besar and not menunda untuk belanja besar?

In Indonesian, verbs like menunda take a direct object without untuk. So:

Natural: menunda belanja besar; menunda membeli (sesuatu); menunda pembelian besar.
Less natural (English-influenced): menunda untuk belanja besar. Prefer to drop untuk.

Is belanja a noun or a verb? Why not berbelanja?

Belanja can be a noun (“shopping/spending”) or a verb in casual Indonesian (“to shop”), but the clearly verbal form is berbelanja.
After menunda you want a noun phrase, so belanja (besar) is perfect. If you wanted a verb idea, you could say menunda berbelanja.

What exactly does belanja besar mean? Would pembelian besar or pengeluaran besar be better?

Belanja besar = “major shopping/spending,” a general idea of big-ticket purchases.
Pembelian besar = “a large purchase/large purchases” (focus on buying transactions).
Pengeluaran besar = “large expenses” (broader: bills, fees, not just shopping). All are fine; pick based on nuance. For “big-ticket purchases,” belanja besar or pembelian besar both work.

Could I say belanja besar-besaran?

Yes, but it changes the meaning. Besar-besaran means “on a large scale.” Belanja besar-besaran suggests a shopping spree or large-scale spending, not just postponing big purchases in general.

Can I move the clauses around?

Yes. Indonesian allows flexible order:

Kalau gaji belum masuk, kami menunda belanja besar.
Kami menunda belanja besar kalau gaji belum masuk. Both are natural. Keep misalnya near the thing it exemplifies: Misalnya, kalau gaji belum masuk, kami menunda belanja besar.

Do I ever need yang in belanja besar?

Not for a simple adjective. Besar follows the noun directly: belanja besar. Use yang for emphasis or to make it restrictive:

belanja yang besar = specifically “the big spending (as opposed to the small).”

What’s the passive or fronted-object version?

Belanja besar kami tunda. (fronted object, still active)
Belanja besar ditunda (oleh kami). (passive) These put emphasis on the spending rather than on “we.”

How formal or informal is this sentence? Any everyday variants?

Neutral. Variants:

More formal: Jika gaji belum masuk, kami menunda pembelian besar.
Very natural casual: Kalau belum gajian, kami tahan dulu belanja besar.
Softening with dulu (“for now”): Kami menunda belanja besar dulu.

Does gaji need a possessor like “my/our”? Why is it just gaji?

Indonesian often omits possessors when obvious. Gaji belum masuk is commonly understood as “my/our salary hasn’t come in (to our account).” You can add it if needed: Gaji saya/kami belum masuk.

How would I ask or confirm this in everyday talk?

Question: Gaji sudah masuk? / Gaji sudah/belum?
Affirmative: Gaji sudah masuk.
Negative (still waiting): Gaji belum masuk.

Is upah the same as gaji?

Both are pay, but:

Gaji is salary (usually monthly, fixed).
Upah is wages (often hourly/daily, piecework). In many contexts people still say gaji generically, but the distinction exists.