The answer is: Honesty, even in the face of personal loss.
Clearly the text reflects upon the value of honesty as a virtue to be pursued. In fact, the story represents this in two different moments. The first one, and most obvious, concerns the little girl who does not hesitate to be honest about having found the bag with the golden coins (even if that meant she would not keep the golden coins); but also, at a second moment, the fisherman, when asked what was he looking for, is also honest about the object of his search and he is not suspicious about the little girl and does not withhold any information, making it seem like he trusts that other people will also be honest.
Honesty, in the end, is prized in showing the fisherman giving a golden coin to the girl in approval of her honesty.