Решение по жалобе сохранили на случай, если взыскание нужно будет аннулировать.

This is a very common Russian pattern.

Сохранили is past tense plural, but Russian often uses the 3rd person plural with no stated subject to mean:

• someone did it
• they did it
• the authorities/people/staff did it

So here it does not necessarily mean a specific they. It is an indefinite-personal construction.

A natural English equivalent is often something like:

• They kept the decision...
• The decision was kept...
• Someone kept the decision...

Russian uses this structure a lot when the doer is unknown, unimportant, or obvious from context.

Because по жалобе is the normal legal/administrative way to say on the complaint / regarding the complaint / in response to the complaint.

Here:

• решение = decision
• по жалобе = on the complaint / concerning the complaint

The preposition по often means regarding, concerning, or on in official language.

So:

• решение по жалобе = decision on a complaint
• ответ по заявлению = response regarding an application
• заключение по делу = conclusion on the case

Using решение жалобы would not sound right here.

Because по here requires the dative case.

The base form is:

• жалоба = complaint

Its dative singular form is:

• жалобе

So:

• по жалобе = on the complaint / regarding the complaint

This is a very useful pattern to remember:

• по вопросу = on the question / regarding the issue
• по делу = in the case / regarding the case
• по заявлению = regarding the application

На случай, если is a set expression meaning:

• in case
• in the event that

Literally, на случай is something like for the case/event, and then если introduces the possible situation.

So the structure is:

• на случай, если + clause

Examples:

• Возьми зонт на случай, если пойдёт дождь.
  Take an umbrella in case it rains.

• Документы сохранили на случай, если они понадобятся.
  They kept the documents in case they are needed.

This sentence uses exactly that same precautionary idea.

Because если introduces a subordinate clause.

In на случай, если взыскание нужно будет аннулировать, the part beginning with если is a dependent clause:

• если взыскание нужно будет аннулировать = if / in case the penalty has to be annulled

Russian normally separates such clauses with a comma.

So the comma here is standard and expected.

This is another very common Russian structure.

Break it down:

• нужно = necessary / it is necessary
• будет = will be
• аннулировать = to annul / cancel

Together:

• нужно будет аннулировать = it will be necessary to annul
• more naturally: will have to be annulled or it will be necessary to cancel

This is an impersonal construction. There is no personal subject like I or they.

Compare:

• нужно сделать = it is necessary to do
• нужно будет сделать = it will be necessary to do
• придётся сделать = it will be necessary / one will have to do

So in your sentence, Russian is literally saying something like:

• if it becomes necessary to annul the penalty

Because будет agrees with нужно, not with взыскание.

The phrase нужно будет аннулировать is an impersonal structure. The core idea is:

• it will be necessary

Since it is impersonal, Russian uses singular будет.

This is similar to:

• Нужно будет подождать. = It will be necessary to wait.
• Нельзя будет войти. = It will not be possible / allowed to enter.
• Можно будет обсудить. = It will be possible to discuss.

So взыскание is the object of аннулировать, not the subject of будет.

Because it depends on нужно будет.

After words like нужно, можно, нельзя, надо, Russian normally uses an infinitive:

• нужно сделать = need to do
• можно войти = may enter / can enter
• нельзя курить = must not smoke

So:

• нужно будет аннулировать = it will be necessary to annul

The action itself stays in the infinitive form.

Because it is the direct object of аннулировать.

The dictionary form is:

• взыскание

This is a neuter noun, and in the singular its nominative and accusative forms are the same.

So:

• nominative: взыскание
• accusative: взыскание

Since аннулировать takes a direct object, we expect the accusative, but for this noun it looks identical to the base form.

In legal or administrative language, взыскание often means some kind of penalty, sanction, or disciplinary measure.

Depending on context, it could be translated as:

• penalty
• sanction
• disciplinary action
• disciplinary penalty

In other contexts, взыскание can also relate to recovery/collection of money or debt, because the verb взыскать can mean to recover/collect by legal means.

So this is a word whose exact English translation depends heavily on context. In your sentence, it likely means an official sanction or penalty that might later need to be cancelled.

Сохранили is perfective, and that fits the meaning well: it presents the action as a completed whole.

Here the idea is that they took the action of keeping/saving/retaining the decision for possible future use.

• сохранили = completed action
• сохраняли = process, repeated action, or background activity

So:

• Решение ... сохранили = They kept/saved the decision.
• Решение ... сохраняли would sound more like they were engaged in preserving it over some period, which is not the point here.

Perfective is the normal choice when reporting a one-time completed administrative action.

Yes, the word order is normal.

Russian word order is flexible, and this version is quite natural because it starts with what happened to the important item:

• Решение по жалобе — topic
• сохранили — action
• на случай, если... — purpose / precaution

So it feels like:

• As for the decision on the complaint, they kept it in case...

Other orders are possible, for example:

• На случай, если взыскание нужно будет аннулировать, решение по жалобе сохранили.

That version puts more emphasis on the precautionary reason first.

The original order is probably the most straightforward in neutral official style.

Usually решение means decision, but in legal or official contexts it can sometimes be translated as:

• decision
• ruling
• determination
• resolution

The best choice depends on the institution and the type of document.

In решение по жалобе, a very natural translation is often:

• decision on the complaint
• decision regarding the appeal/complaint

If the context is more formal or judicial, ruling on the complaint might also work.

Yes, but the meaning would shift slightly.

• если alone = if
• на случай, если = in case

That difference matters.

Compare:

• Сохранили, если понадобится — less natural here, and sounds more like a condition
• Сохранили на случай, если понадобится — clearly expresses precaution

So на случай, если shows that they kept the decision as a safeguard, because this situation might arise later.

That preventive nuance is exactly why the longer expression is used.