I was at this supermarket near my university about 20H30. 
When the person at the counter starts billing my purchase, zhe said *bonjour* when I was expecting a *bonsoir*. My response was an immediate *bonsoir* (by the books!), and I felt it made them a bit uneasy.

Again, once at the fish market in the same supermarket, another person greeted with me *bonjour*, this time I replied *salut* which was a desperado attempt. 
Both incidents was in the middle of December night, about 20H30.

Is it fine to wish someone *bonjour* in the night? If so what would be an appropriate reply?

Addendum 1 - This situation is not a synonym to [unclear time scenario](https://french.stackexchange.com/questions/6401/quand-dire-bonjour-et-quand-dire-bonsoir). This is rather an exception to the rule, than a confusing application.