daf bit: Kiddushin 62

[cellio]

A mishna on today's daf teaches: if a man betroths a woman thinking she is the daughter of a kohein and it turns out she's the daughter of a levite only, or the reverse; or he thought she was poor and she is actually rich, or the reverse -- she is betrothed, because she did not deceive him. (He failed to properly investigate what he thought to be true.) On the other hand, if a man says: be thou betrothed... after I become a proselyte, or after you become a proselyte, or after you are liberated, or after your husband dies -- in all these cases she is not betrothed. The g'mara explains why not -- these are actions that are not in his power. But, the g'mara asks, surely becoming a proselyte is in his power? No, because you need the approval of a beit din, a court of three judges. (62a-b)
The g'mara does not here comment that in order to say "after you become a proselyte", the man must be betrothing a non-Jewish woman (at the time). I don't know if a betrothal to a future Jew would be held to be valid, or if the discussion just never gets that far because of the other issue raised.