Jeg vil købe lampen, når den kommer på lager igen.

It can suggest both, but the nuance is usually want/intend to buy rather than a completely neutral future.

In Danish, vil often shows willingness, intention, or decision. So Jeg vil købe lampen sounds like I want to / I intend to buy the lamp.

For a simple future statement, Danish very often just uses the present tense: Jeg køber lampen i morgen = I’m buying / I’ll buy the lamp tomorrow

So in this sentence, vil adds the idea that the speaker has the intention or desire to buy it once it is available again.

Because vil is a modal verb, and modal verbs are followed by the infinitive in Danish.

So:

• vil købe = want to buy / will buy
• kan købe = can buy
• skal købe = must / will buy

You do not add at after a modal verb here. So:

• Jeg vil købe lampen = correct
• Jeg vil at købe lampen = incorrect

Because lampen means the lamp, while en lampe means a lamp.

Danish usually marks definiteness by adding an ending to the noun:

• en lampe = a lamp
• lampen = the lamp

So the sentence is talking about a specific lamp, not just any lamp. That specific lamp is already known from the context.

Because lampe is a common-gender noun, and common-gender nouns are referred to with den.

In Danish:

• common gender: den
• neuter gender: det

So:

• en lampe → lampen → den
• et hus → huset → det

Since lampe is en lampe, the correct pronoun is den.

Because når means when, while hvis means if.

Use når when the event is seen as something that is expected or treated as a real future occurrence:

• Jeg vil købe lampen, når den kommer på lager igen
  = I will buy the lamp when it comes back in stock

Use hvis for a condition:

• Jeg vil købe lampen, hvis den kommer på lager igen
  = I will buy the lamp if it comes back in stock

So når suggests the speaker expects the lamp to come back in stock. Hvis would make it sound more uncertain.

Because Danish often uses the present tense for future meaning, especially in time clauses introduced by words like når.

So når den kommer på lager igen is completely normal Danish and works like English:

• when it comes back in stock

This is similar to English, where we also usually say:

• I’ll buy it when it comes back in stock not
• I’ll buy it when it will come back in stock

So the present tense after når is the expected form.

På lager is a fixed expression meaning in stock.

So:

• varen er på lager = the item is in stock
• kommer på lager igen = comes back into stock / is back in stock again

Word-for-word, it may look like on stock, but that is not how it works in English. The natural translation is in stock.

That is the most natural placement here.

Igen means again, and in sentences like this it often comes late in the clause:

• når den kommer på lager igen

This sounds very natural in everyday Danish.

You may also see other placements, but they can sound more formal or marked:

• når den igen kommer på lager

That version is possible, but the original sentence is the more natural everyday order.

Because når den kommer på lager igen is a subordinate clause, and Danish often places a comma before subordinate clauses.

So the sentence is divided into:

• main clause: Jeg vil købe lampen
• subordinate clause: når den kommer på lager igen

You should know, though, that Danish has had more than one accepted comma system. Because of that, learners may see both:

• Jeg vil købe lampen, når den kommer på lager igen
• Jeg vil købe lampen når den kommer på lager igen

The version with the comma is very common and completely standard to learn from.

Then the word order in the main clause changes, because Danish follows the verb-second rule.

So you get: Når den kommer på lager igen, vil jeg købe lampen.

Notice the change:

• normal main clause: jeg vil købe
• after a fronted clause: vil jeg købe

That inversion is very important in Danish. If you begin with the time clause, the finite verb in the main clause comes before the subject.

Yes, and that is very natural Danish.

That version uses the present tense with future meaning:

• Jeg køber lampen, når den kommer på lager igen

It often sounds a bit more neutral and matter-of-fact than jeg vil købe, which can sound more like intention or willingness.

So both are possible, but they are not exactly identical in nuance:

• jeg vil købe = I intend / want to buy
• jeg køber = I’ll buy / I’m buying

Both are common and correct depending on what you want to emphasize.