The gemara says that R' Meir and Chachamim argue about milva b'shtar, when he is mekadesh her with money he is owed by someone else whether she would be mekudeshes, and offers a few suggestions as to what the point of argument is. One of the suggestions is that they argue about whether one who sells a shtar chov has rights to be mochel it. According to R' Meir she is mekudeshes because he doesn't retain the ability to be mochel after being makneh it to her as kesef kiddushin, and Chachamim says he retains the ability to be mochel so she isn't convinced that she will be able to keep the money (no semichas da'as) so she isn't mekudeshes. Tosfaos explains that the ability to be mochel a shtar chov even after selling it off is based on the assumption that the sale is only binding m'drabonon but not m'doraysa (See Ritvah here and Ran in kesubos who offer a different interpretation that he retains a shi'bud haguf and is only selling the shi'bud ma'mon).
The gemara also suggests that they argue by a milva al peh that he was makneh to her through ma'amad shelashtan. R' Meir holds that ma'amad shelashtan works so that she is mikudeshes and the chachamim say that ma'amad shelashtan doesn't work on a loan.
The question is whether according to R' meir where the selling of a shtar chov is only d'rabonon and the kinyan of ma'mad shelashtan is only d'rabonon, would she be mikudeshes only d'rabonon or even m'doraysa?
The Avnei Miluim (28:33) discusses at length the issue of kinyan d'rabonon working m'doraysa based on hefker beis din. He quotes Rabbeinu Yerucham who says that ma'amad shelashtan will only create a kidushin d'rabonon, but the beis yosef argues and says she would be mikudeshes m'doraysa. According to the Beis Yosef that by ma'mad shelashtan the kiddushin would be d'oraysa, would it also be d'oraysa by being makneh to her a shtar chov (according to tosafos that the kinyan is only d'rbonon)?
The Avnei Milum offers a sevara that by selling shtaros where the seller retains the ability to be mochel because the kinyan is only d'rabonon, the kinyan d'rabonon cannot transform into a kinyan d'oraysa because that will prevent mechilla, so we have to assume that chazal never strengthened the kinyan d'rabonon to be d'oraysa using hefker beis din. But, regarding ma'mad shelashtan it is possible that the rabbonon completely transformed the kinyan d'rabonon into a d'oraysa and would therefore create a kiddushin d'oraysa.
This would be very consistent with Tosafos who say that by the shtar chov, mechila would work because the kinyan is only d'rabonon, but by ma'mad shelashtan Tosafos says that mechila would not work. Since the mechila would not work for the ma'mad shelashtan, it is very possible that chazal transformed the kiddushin d'rabonon into a kiddushin d'oraysa.