? ???????????, ???-??????? ?????? ??????, ????? ???-??????? ?????????? ?????????, ??????? ??????????; ????????, ?????? ???????, ????? ???-??????? ????????? ????????, ??????????. ? ??????? ??????? ???????? ???????? ?????-??????????, ?????????, ??????????? ????????? ?????????? ??????????, ???????? ??? ??????? ????????? ?????? ??????????? ?????; ?????????? ???????????, ?????????. ? ?????-??????? ????????, ??????? ??????????, ?????????? ?????????, ??? ??????? ????????? ?????? ??????????? ?????; ???????????? ?????? ??????????, ???????? ?????????
apushatayid, you assume that the shidduch is more valuble than the correct usage of Eretz Yisroel by a shevet.
Each shevet had its mission within the Klal, and Menashe thought it would be to the detriment of their mission to lose the nachala. And Hashem agreed.
Besides, its not “their” property (as it belonged to Bnos Tzlafchad).