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 apparently explained at length in a Sicha of Nasso 5741, but I couldn't find it in the online transcript.
*As far as I know, this idea was an invention of De Beers