At first glance, this seems to be a similar case to
proceeds, which I considered some time back. (Similar even in meaning!) But actually
earnings has a stronger case than
proceeds, which I accepted anyway.
Proceeds could either be an inflection of the verb
proceed - "She proceeds to the check-out" - or a plural noun with no currently-used singular form - "the proceeds of crime".
However, in this case, the original verb is not
earning; it is
earn. There is no widespread usage of
earnings other than as the plural noun meaning income or profit, which has no singular form in current usage. So I will add earnings as an allowable word, rhino.