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According to this page:
https://en.wiktionary.org/wiki/%D0%BA%D1%80%D0%B0%D1%81%D0%BD%D1%8B%D0%B9
красен is a short for of красный.

According to Google красен can mean:
1) is red
2) red in the...

These two sentences are supposed to mean basically the same thing:
Это был случай одного красен.
Это был случай услуга за услугу.
Both can be translated to
"It was a case of one good turn...