
die Nebensache
side issue
A matter, detail, or aspect that is unimportant or of lesser significance compared with other things.
Das Geld ist bei dieser Entscheidung nur eine Nebensache.
Money is only a side issue in this decision.
side issue
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side issue
A matter, detail, or aspect that is unimportant or of lesser significance compared with other things.
Das Geld ist bei dieser Entscheidung nur eine Nebensache.
Money is only a side issue in this decision.
The word is made up of the preposition 'neben' (beside/next to) and the noun 'die Sache' (thing/matter). So it literally describes a 'thing that lies beside it' and is therefore less important.
Für ihn ist Pünktlichkeit die Hauptsache, alles andere ist Nebensache.
For him, punctuality is the main thing; everything else is a side issue.
'Nebensache' (side issue) is very often used as a predicate noun with the verbs 'sein' (to be) or 'werden' (to become). The pattern is: [something] is/becomes (only) a side issue.
Als sie die gute Nachricht hörte, wurden alle ihre Sorgen zur Nebensache.
When she heard the good news, all her worries became a side issue.
The expressions 'zur Nebensache werden' (to become a side issue) or 'zur Nebensache machen' (to make something a side issue) are very common. They express that something loses importance because something else comes to the foreground.
Angesichts der ernsten Lage wurde der Streit über Kleinigkeiten schnell zur Nebensache.
Given the serious situation, the argument about small things quickly became a side issue.
detail
Both words can refer to a subordinate aspect of a larger topic. “Das Detail” mainly describes something as a small part, while “die Nebensache” also indicates that this part is less important. Use “Detail” neutrally and “Nebensache” when its limited importance is being emphasized.

Ob der Raum blau oder weiß ist, bleibt bei dieser Entscheidung eine Nebensache.
Whether the room is blue or white is a side issue in this decision.
Das kleine Detail im Vertrag müssen wir vor der Unterschrift noch prüfen.
We still need to check the small detail in the contract before signing it.