Learn Hardcore Japanese: ryougae ni ha genkin ga hituyouna noni, kare ha genkin wo wasureta. - Even though cash is necessary for exchanging money, he forgot his cash.

に marks the domain/purpose: “for/in the context of.”
は topicalizes that phrase: “as for …” So 両替には means “as for currency exchange / for exchanging money,” introducing the condition under which something is needed. This is a very common requirement pattern: Xには Yが必要だ (“For X, Y is necessary”).
Examples: 日本に行くにはパスポートが必要です。, 研究にはお金が必要だ。

You could say 両替に現金が必要だ too (grammatical: “cash is necessary for exchange”), but には makes “currency exchange” the topic and often sounds more natural when stating general requirements.

Why is it 現金が必要な and not 現金が必要だ before のに?

Because のに attaches to the attributive (noun‑modifying) form. With nouns and na‑adjectives, だ becomes な before の:

Noun: 学生だ → 学生なのに
Na‑adj: 静かだ → 静かなのに
Here: 必要だ → 必要なのに

So 現金が必要なのに is the correct form (your spacing shows 必要なのに, but functionally it’s 必要なのに).

What nuance does のに add here?

のに means “even though/although,” and it carries a stronger sense of contradiction and often the speaker’s frustration or surprise. It implies “Given X, Y shouldn’t have happened, but it did.”

Milder alternatives: けど／が／けれども (“but,” less emotional).
More formal: にもかかわらず.
More accusatory/complaining: くせに (use with care; it can sound blaming).

Why is it 現金が in the first clause but 現金を in the second?

現金が必要: が marks the subject of 必要 (“necessary”). Literally “Cash is necessary.”
現金を忘れた: 忘れる is a transitive verb, so the thing forgotten takes を (“he forgot cash”).

Could I use いる instead of 必要?

Yes. 現金が要る（いる） means “cash is needed” and is more casual/conversational than 必要だ.

Plain: 現金が要るのに…
Polite: 現金が要りますが／のに… Note the kanji for this verb is 要る (to need), not the existence verb 居る.

Do I need to say 両替するには instead of 両替には?

Both work:

両替には現金が必要だ: “For currency exchange, cash is necessary” (general statement).
両替するには現金が必要だ: “In order to exchange (money), cash is necessary” (focuses on the act).
The second explicitly frames it as “to do X, you need Y,” but the meaning is essentially the same.

Why 彼は and not 彼が?

は marks the topic (“as for him…”), which is natural in the main clause after a subordinate clause. 彼が would put focus on “he” as the subject (“it was he who forgot”), adding emphasis or contrast. Both are grammatical; 彼は is the default here.

Can I drop the second 現金 or replace it with それ?

Yes, depending on context:

Omit: …のに、彼は忘れた。 (If it’s crystal clear what was forgotten.)
Use pronoun: …のに、彼はそれを忘れた。 (Possible, but Japanese often prefers repeating the noun to avoid ambiguity.)
Often even more natural: …のに、彼は現金を持ってくるのを忘れた。 (“…he forgot to bring cash.”)

Why is it 現金が必要 and not 現金は必要?

現金が必要 identifies what is necessary (neutral/new information).
現金は必要 topicalizes/contrasts: “As for cash, (at least) it’s necessary,” possibly implying contrast with other things (e.g., “Cash is necessary, but ID isn’t”). In a plain requirement statement, が is standard.

Could I use で instead of に after 両替?

No in this structure. で marks place/means (“at/with”), e.g., 銀行で両替する (“exchange at the bank”).
Here you want the domain/purpose marker に: 両替には現金が必要だ (“For exchange, cash is necessary”). 両替で現金が必要 sounds off.

How would I make the sentence polite?

Keep the subordinate clause plain, and make the main clause polite:
両替には現金が必要なのに、彼は現金を忘れました。
Or use a milder “but” in polite speech:
両替には現金が必要ですが、彼は現金を忘れました。
Note: のに is fine in polite sentences; the predicate before のに stays in plain form.

What’s the difference between 両替, 換金, and 交換?

両替: currency exchange or changing denominations (e.g., dollars ↔ yen; bills ↔ coins).
換金: converting something into cash (e.g., selling points/gift cards/crypto for money).
交換: exchanging one item for another; not used for currency exchange in this sense.

Does のに always sound blaming?

Not always, but it often carries an emotional “contrary to expectation” tone. It can be used about yourself (self‑frustration): こんなに準備したのに、失敗した.
If you want a neutral “but,” prefer けど／が. If you want strong blame, くせに is harsher than のに.

Is 忘れた just “forgot,” or does it imply “forgot to bring”?

In contexts like this, 現金を忘れた typically means “forgot (to bring) cash.” To be explicit, you can say:

現金を持ってくるのを忘れた。
Add regret/”ended up”: 現金を忘れてしまった。／持ってくるのを忘れてしまった。

Why are there spaces between the Japanese words?

They’re pedagogical spacing for learners. In normal Japanese writing, you wouldn’t insert spaces:
両替には現金が必要なのに、彼は現金を忘れた。

How do you read and parse the sentence?

両替 (りょうがえ) noun/suru‑noun “money exchange”
に particle (domain/purpose)
は topic marker
現金 (げんきん) noun “cash”
が subject marker
必要 (ひつよう) na‑adjective/noun “necessary/need”
な attributive form of だ before の
のに concessive conjunction “although/even though”
、 comma
彼 (かれ) pronoun “he”
は topic marker
現金 (げんきん) noun
を object marker
忘れた (わすれた) past plain of transitive verb 忘れる “to forget”