Learn Hardcore Japanese: kare ha mukou de ryougae wo sitano ni, genkin ga tarinai to itte iru. - He exchanged money over there, yet he says he doesn’t have enough cash.

のに marks a strong, contrary-to-expectation contrast: “even though/despite.” It often carries a nuance of surprise, frustration, or complaint by the speaker.
けど is a softer “but/though” and can simply link two facts without implying such a strong contradiction.
So ...両替をしたのに、... feels like “He went to the trouble of exchanging money, yet…!” whereas ...両替をしたけど、... is more neutral.

Is the の in したのに the nominalizer? When do I use なのに?

Here のに is one conjunctive particle meaning “although/even though.” The の is not the standalone nominalizer.
After verbs and i‑adjectives you use just ...のに: したのに, 高いのに.
After nouns and na‑adjectives you need なのに: 学生なのに, 静かなのに.

Why 向こうで and not 向こうに or あそこで?

で marks the place where an action happens. He did the exchanging at that location: 向こうで両替をした.
に marks destination/arrival or the location for certain verbs of existence; 向こうに両替をした is unnatural. You’d say 向こうに行って両替をした (“went there and exchanged”).
あそこで = “at that place over there (specific/visible).” 向こうで is vaguer: “over there/on the other side/at the other place.”

Does 両替 mean foreign currency exchange or just breaking a bill?

両替 covers both: exchanging currencies and changing denominations (e.g., breaking a 10,000‑yen note).
For specifically breaking a bill, people also say お札を崩す.
Don’t confuse with お釣り (the change you receive after paying).

Why is there an を after 両替? Can I drop it?

両替 is a する‑noun. Both 両替をする and 両替する are standard. So 両替をした and 両替した are both fine.
Using を can sound a bit more careful/formal; without を is slightly leaner.

Why use 現金 instead of お金?

現金 = “cash” (notes/coins). お金 = “money” in general (including bank balance, electronic money).
現金が足りない implies the person lacks physical cash, even if they might have money in other forms.

Why is it 現金が足りない with が, not は?

足りる is an intransitive verb; the thing that is (not) sufficient takes が: 水が足りない, 時間が足りる.
Using は would add a contrast/topic nuance: 現金は足りない (“As for cash, it’s not enough [but something else might be]”). が is the default for stating the fact.

What exactly does 足りない mean here? Is 足らない okay? How about formal alternatives?

足りない is the negative of 足りる: “to be insufficient/not enough.”
足らない is a common variant (often heard in set phrases); 足りない is more standard.
A formal alternative is 不足している (“is insufficient”), e.g., 現金が不足している.

What nuance does と言っている add compared with と言う or と言った?

と言っている reports what someone is saying now or repeatedly; it can present a current stance/claim: “is saying/keeps saying/claims.”
と言う (plain non‑past) can state a general fact or immediate utterance; と言った reports a completed past statement (“said”).
Polite form: と言っています.

Who is the subject of 言っている? Is it automatically 彼?

By default, yes—the subject continues from the topic 彼は unless context shifts it.
Japanese often omits repeated subjects. If a different person were speaking, you’d mark that explicitly (e.g., 店員が…と言っている).

Can I use って instead of と in と言っている?

Yes. 現金が足りないって言っている is very natural and casual.
って is the colloquial quotative; prefer と in neutral/formal writing or when you want a crisper tone.

How would the tone change if I used けど instead of のに? Any other ways to tweak the tone?

With けど: 向こうで両替をしたけど、現金が足りないと言っている。 Softer, more neutral contrast, less “Why on earth?!” feel.
To add an explanatory/emphatic tone, you can add んだ/のだ outside or inside the quote:
- Outside: …と言っているんだ。 (speaker’s explanation)
- Inside the quote: 現金が足りないんだ (his emphatic claim)
A more formal “despite” is にもかかわらず.

Is using 彼 natural here?

It’s grammatical, but everyday Japanese often drops third‑person pronouns or uses names/titles instead. Depending on context, 彼 can also suggest “boyfriend.”
In conversation you might hear simply: 向こうで両替をしたのに、現金が足りないと言っている。 (subject understood) or use a name: 田中さんは…