Das Guthaben ist niedriger als die Nachzahlung.

Guthaben usually means a credit balance, positive balance, or amount in your favor.

In financial/billing contexts, it is the money that is available to you or the amount a company owes you.

So in this sentence, das Guthaben is the amount of credit, and it is being compared with die Nachzahlung.

Nachzahlung means an additional payment, back payment, or amount that still has to be paid later.

It is common in bills, taxes, rent, utilities, and similar contexts. If the final calculation shows that you did not pay enough before, you may have a Nachzahlung.

So the sentence compares:

• Guthaben = credit/balance in your favor
• Nachzahlung = extra amount you still owe

Because German nouns have grammatical gender, and these two nouns have different genders:

• das Guthaben = neuter
• die Nachzahlung = feminine

This is something you usually have to learn together with the noun.

A helpful tip:

• many nouns ending in -ung are feminine, so die Nachzahlung follows a very common pattern.
• Guthaben is neuter, so it takes das.

Yes. niedriger is the comparative form of niedrig, so it means lower.

• niedrig = low
• niedriger = lower

In English, we might say lower than or sometimes less than. In German, niedriger is very natural when comparing amounts, prices, levels, or values.

So:

• Das Guthaben ist niedriger = The credit balance is lower

Not in the usual sense. Here, -er is mainly the comparative marker.

• niedrig = low
• niedriger = lower

After sein, adjectives are normally used in their basic predicate form and do not take normal adjective endings for gender/case.

So niedriger is not agreeing with das Guthaben the way an attributive adjective would. It is simply the comparative form of niedrig.

Compare:

• ein niedriger Betrag = a low amount
  
  • here niedriger is an adjective with an ending before a noun
• Der Betrag ist niedriger = the amount is lower
  
  • here it is a predicate adjective in the comparative

In this sentence, it is the second type.

Als is used for comparisons of inequality, meaning than.

So:

• niedriger als = lower than

This is the normal pattern in German:

• größer als = bigger than
• kleiner als = smaller than
• teurer als = more expensive than
• niedriger als = lower than

So Das Guthaben ist niedriger als die Nachzahlung literally means: The credit balance is lower than the additional payment.

Because German distinguishes between:

• so ... wie for equality
• comparative + als for difference

Examples:

• Das Guthaben ist so hoch wie die Nachzahlung. = The credit balance is as high as the additional payment.
• Das Guthaben ist niedriger als die Nachzahlung. = The credit balance is lower than the additional payment.

So:

• wie = as
• als = than

Sometimes yes, but they are not always equally natural.

• niedriger is very common for amounts, levels, prices, rates, and values
• geringer can also work for quantities or amounts and often sounds a bit more formal
• kleiner is possible, but with money/amounts it is often less idiomatic than niedriger or geringer

So in a billing or financial sentence, niedriger sounds very normal.

Because they are nouns, and all German nouns are capitalized.

So:

• das Guthaben
• die Nachzahlung

This is one of the most noticeable spelling rules in German.

The structure is very straightforward:

• Das Guthaben = subject
• ist = verb
• niedriger = predicate adjective in the comparative
• als die Nachzahlung = comparison phrase

So the pattern is:

Subject + sein + comparative adjective + als + thing compared

This is a very common German pattern.

Here it is singular.

Das Guthaben refers to one balance or one credit amount as a whole.

Even though it may represent money, German treats it as a singular noun in this sentence:

• Das Guthaben ist ...

In a sentence like this, learners usually do not need to think of als as forcing a special case the way a preposition might.

The important thing is that als introduces the second part of the comparison:

• niedriger als die Nachzahlung

Because the compared item is just a noun phrase here, you simply use the natural form:

• die Nachzahlung

So for practical learning, the key rule is: comparative + als + noun phrase

Yes. You could also say:

Die Nachzahlung ist höher als das Guthaben.

That means the same basic thing from the opposite perspective:

• The additional payment is higher than the credit balance.

German often allows this kind of rephrasing, depending on what you want to emphasize.