You're right to think there might be a difference between the two - earning is the present participle of the verb earn, and earnings is a plural noun; see e.g. http://www.ldoceonline.com/dictionary/earnings . Earning in the singular isn't a noun.

Earnings is the word you want here.

Danke, Villager. Warum hat dann aber Leo zwei Einträge für das Nomen Ertrag - earning + earnings?

Having looked at the Ertrag entry, I'd say it's wrong to have earning as a noun to translate Ertrag? The entry under earnings seems fine, but when you look at earning Siehe Wörterbuch: earning the examples of Substantive actually seem to me to show the participle being used as an adjective.

I don't know if we've got a falscher Eintrag here.