Are there any Halachic issues with giving a girl a ring while asking her to marry him? Are there any sources that discuss the sha'alah of safek Kiddushin, etc?
The Lubavitcher Rebbe specifically discouraged giving a ring at any time during the engagement. (The practice in Chabad is to give the diamond ring* in the Cheder HaYichud after the Chuppa, and even other gifts are not given directly, or at least not with witnesses).
This is apprently explained at length in a Sicha of Nasso 5741, which unfortunately is not available online, and I don't have a copy of the book.
*As far as I know, this ideas was an invention of De Beers