The Gemora Kidushin on Daf 63 learns :
If you hold "Yeshno Lischirus Mitchila vad Sof", then a debt has built up while the man was doing work for the woman and this is Mekadesh bemilve and not mekudeshes.
However, if you hold "Eino Lischirus ela Lebusof", then nothing is owed until the man has completed the job for the woman and this is not Mekadesh bemilve and therefore mekudeshes.
Even if the hischayvus to pay only comes at the end, why is she mekudeshes? The reason that Mekadesh bemilve is not mekudeshes is that milva lehotzaa nitno, and therefore he is not giving her anything tangible for kidushin. If the hischayvus to pay only comes at the end, even if we don't call this a "debt", he still has not given her anything tangible?
Regards,
Yisroel Alter Pacanowski
Dear Isi,
The Kovetz Shiurim ( Baba Kama 144) by R' Elchonon Vaserman asks your question and answers that he gives her now the pleasure of his finished act and instead of pay, he uses it for marriage. But if Yeshno meTechilah veAd Sof she already owes him from before so he is not giving her now the Hanaah since it's considered already hers.
The Pnei Yehoshua on Daf 48b explains differently. Normally Mekadesh b'Milveh doesn't work, but you could marry her with the Hanaah of being Mochel the debt, if he explicedly says so (otherwise he means to use the Milveh itself which is no good). But here there is no Milveh yet because Eino Ela baSof. So even without specifying, he is obviously using for Kidushin the Mechilah of the about to start Milveh- and that works- without openly saying b'Hana'as Mechilas Milveh.
All the best,
Reuven Weiner