Le client paie l’impayé lorsque la facture arrive.

Because le becomes l’ before a vowel sound. This is called elision.

• le client
• la facture
• l’impayé

French does this to make pronunciation smoother. Since impayé begins with i, you must write l’impayé, not le impayé.

Here, impayé is being used as a noun.

As an adjective, impayé means unpaid:

• une facture impayée = an unpaid bill

But in your sentence, l’impayé means something like:

• the unpaid amount
• the overdue payment
• the outstanding debt

So even though it originally comes from an adjective, here it functions as a thing: the money that has not been paid.

Paie is the je / il / elle / on form of the verb payer in the present tense.

• je paie
• tu paies
• il/elle/on paie
• nous payons
• vous payez
• ils/elles paient

A useful thing to know: with payer, you may also see forms like paye. Both spellings are accepted in many contexts:

• je paie / je paye
• il paie / il paye

So Le client paie is completely correct.

French often uses the present tense for:

• general truths
• habitual actions
• actions in a sequence
• events that are understood as regular or typical

So Le client paie l’impayé lorsque la facture arrive can describe a normal pattern: whenever the invoice arrives, the customer pays the unpaid amount.

This is similar to English sentences like:

• The customer pays when the invoice arrives
• Water boils at 100°C

Even if the action happens in the future in a real situation, the present tense can still be used to describe the rule or routine.

Both can mean when.

• lorsque = a bit more formal or written
• quand = more common in everyday speech

So:

• lorsque la facture arrive
• quand la facture arrive

Both are correct. In this sentence, lorsque gives a slightly more formal tone.

La facture uses the definite article because it refers to a specific invoice that is understood from the context.

French often uses definite articles where English might also use the, but sometimes French uses them more naturally than English does.

Compare:

• la facture = the invoice / the bill
• une facture = an invoice / a bill (any one, not specific)

Here, la facture suggests a particular invoice relevant to the situation.

Yes. In French, arriver is commonly used for things like letters, packages, messages, and bills when they are received or show up.

So la facture arrive means something like:

• the bill arrives
• the invoice comes in
• the invoice is received

This is natural French.

In French, after words like quand and lorsque, the present tense is often used when talking about a general or repeated event.

So French says:

• lorsque la facture arrive

not:

• lorsque la facture arrivera in this kind of general statement

If you are talking about one specific future event, French can use the future too, but in a sentence describing a regular pattern, the present is very normal.

A simple guide is:

• l’ = like l
• im = nasal sound, roughly like an in French nasal pronunciation
• pa = pah
• yé = yay but shorter and tenser

A rough English-friendly approximation is:

lan-pay-yay

But the first vowel is nasal, so it is not a full lan sound in real French.

The final -é is pronounced clearly, like ay in say, but without the English glide being too strong.

French word order here is the standard:

Subject + verb + object

• Le client = the subject
• paie = the verb
• l’impayé = the direct object

So the structure is:

Le client / paie / l’impayé
The customer / pays / the unpaid amount

Then the time clause comes after that:

lorsque la facture arrive = when the invoice arrives

Yes, depending on context.

Facture can often be translated as:

• invoice
• bill

A few rough tendencies:

• invoice sounds a bit more formal or business-like
• bill is often more everyday English

So la facture arrive could be:

• the invoice arrives
• the bill arrives

The best choice depends on the situation.

Yes, it sounds fairly formal, mainly because of:

• le client
• l’impayé
• lorsque
• la facture

This is the kind of wording you might see in:

• accounting
• customer payment policies
• business correspondence
• legal or administrative writing

In more everyday French, people might choose simpler wording depending on the exact meaning.