Steve said
Richie,
I've already explained all of this. The inheritance is a side-issue anyway. The daughter marrying a servant was sufficient motive and is the same motive in endless other similar ballads. He wasn't getting a fourth share! He was to get her share. Look at my explanation of the meaning of 'the same'.

Richie then said
I'm not sure about significance of the inheritance it seems that there's more to it than we can know from the text. If we assume that the broadside was written from a translation of Boccaccio then there's nothing really to investigate, the daughter got a portion and wanted the factor to receive the same.

I'm with Steve on this. The expression "the same" is still sometimes used in formal contexts to signify "the aforementioned". I don't take it to mean another portion of the same size. The story is that the daughter got her portion and wanted the factor to receive it (together with, crucially, herself!).