Der Abschlag ist nicht plausibel, zumal die Preisgarantie gilt.

Der Abschlag is in the nominative case because it is the subject of the sentence.

• Der Abschlag = the thing being talked about
• ist = is
• nicht plausibel = not plausible

After sein, German often links a subject to a description:

• Der Abschlag ist hoch.
• Der Abschlag ist nicht plausibel.

So Abschlag stays nominative: der Abschlag.

Abschlag has several meanings in German, so learners often notice that it can be confusing.

Common meanings include:

• deduction
• discount / reduction
• installment payment in some contexts
• tee shot in golf

In this sentence, it most likely means something like a deduction, reduction, or discount, depending on the context you were given.

So even if the overall meaning is already known, it is useful to remember that Abschlag is a context-dependent noun.

Because plausibel is being used predicatively, not attributively.

Compare:

• ein plausibler Abschlag → attributive adjective, so it takes an ending
• Der Abschlag ist plausibel → predicative adjective, so no ending

After verbs like:

• sein = to be
• werden = to become
• bleiben = to remain

German adjectives usually do not take endings.

So:

• Der Abschlag ist plausibel.
• Der Abschlag ist nicht plausibel.

are both correct.

Here, nicht negates the adjective plausibel.

So the structure is:

• ist plausibel = is plausible
• ist nicht plausibel = is not plausible

This is the normal placement when nicht is negating an adjective or adjective phrase at the end of the clause.

So the sentence is saying that the deduction/reduction is not plausible, not negating the noun itself.

Zumal means something like:

• especially since
• particularly since
• all the more so because

It introduces an additional supporting reason, often one that makes the first statement sound even stronger.

So the logic is:

• Der Abschlag ist nicht plausibel = The deduction is not plausible
• zumal die Preisgarantie gilt = especially since the price guarantee applies/is valid

Compared with weil, zumal often sounds a bit more like: and this is even more true because ...

Yes, in many cases you could replace it with weil, but the nuance changes.

• weil = a normal, straightforward reason
• zumal = an extra, strengthening reason

Compare:

• Der Abschlag ist nicht plausibel, weil die Preisgarantie gilt.
  = The deduction is not plausible because the price guarantee applies.

• Der Abschlag ist nicht plausibel, zumal die Preisgarantie gilt.
  = The deduction is not plausible, especially since the price guarantee applies.

So zumal adds a sense of this makes the argument even stronger.

Because zumal introduces a new clause, and in standard German it is separated by a comma.

So:

• Der Abschlag ist nicht plausibel, zumal die Preisgarantie gilt.

That comma helps mark the second clause as a reason or supporting comment.

Yes, it is generally treated like a subordinate clause.

With subordinate clauses, the finite verb normally goes to the end:

• ..., weil die Preisgarantie gilt
• ..., da die Preisgarantie gilt
• ..., zumal die Preisgarantie gilt

In this particular sentence, the clause is very short:

• die Preisgarantie = subject
• gilt = verb

Because there are only two elements, it may not look very different from a main clause, but gilt is still in the expected final position.

Gilt is the 3rd person singular form of gelten.

Here, gelten means:

• to apply
• to be valid
• to be in force

So:

• die Preisgarantie gilt = the price guarantee applies / is valid

This is a very common use of gelten with rules, prices, conditions, guarantees, tickets, laws, and offers.

Examples:

• Das Angebot gilt nur heute. = The offer is valid only today.
• Die Regel gilt für alle. = The rule applies to everyone.

Because German often forms compound nouns by joining words together.

• Preis = price
• Garantie = guarantee

Together:

• Preisgarantie = price guarantee

This is completely normal in German. English often writes such combinations as two words, but German usually combines them into one noun.

A useful rule: the last part of a German compound noun determines the gender and the core meaning.

Since Garantie is feminine:

• die Garantie
• die Preisgarantie

The article is die because the head noun is Garantie, and Garantie is feminine.

In German compounds, the final element decides the gender:

• der Preis
• die Garantie
• therefore die Preisgarantie

So even though Preis is masculine, the full compound is feminine because the last part is Garantie.

Yes, very natural—especially in more formal, business, legal, or administrative contexts.

Plausibel means something like:

• believable
• reasonable
• logically understandable

So nicht plausibel can mean:

• not convincing
• not credible
• not reasonable
• not logically supported

In everyday speech, people might sometimes use alternatives like:

• nicht nachvollziehbar
• nicht verständlich
• ergibt keinen Sinn

But nicht plausibel is perfectly normal and quite common in formal German.