Ibrahim Nakha'i (may Allah have mercy on him) said: "If a man marries a woman and does not specify a dowry, then gives her a gift and the woman accepts it, and consummation takes place, then if the woman later demands the dowry, her demand is not valid if she has already been pleased (with what was given)."
Hadith Referenceسنن سعید بن منصور / كتاب النكاح / 1981