Is it permissible for a girl to give a portion of her dowry as charity before marriage and complete the rest afterward, fearing what people might say, and is her action in doing so correct?

A person's intention to donate something does not obligate them to donate it, because intention is not a vow. Therefore, the questioner is not obligated to spend her dowry in the way of Allah. However, if she wishes to fulfill her intention and is of sound mind, she has the right to dispose of her dowry through charity, gift, or otherwise. The wife has the right to dispose of her dowry by selling, gifting, or giving it as charity, by consensus.

However, if it is the custom of the land that women's provisions are prepared from their dowries, and the questioner has no other source to prepare herself for her husband besides her dowry, then she is obligated to prepare herself for her husband from her dowry, and she does not have the right to dispose of it in a way that violates the condition or custom.