Less than 2 weeks ago we discussed the pasuk in Parshas Re'eh "lo tuchal l'echol b'she'arecha ma'aser degancha..." (12:17). Although "lo tuchal" means you cannot, i.e. it is impossible to do, the pasuk means b'pshuto (as Rashi explains) "you may not", i.e. there is a prohibition of doing so. (In the previous post we discussed why the pasuk did not simply word it as a prohibition.)
The strange construction of "cannot" appears again in our parsha -- "lo yuchal l'vaker es ben ha'ahuva", you cannot give first-born rights to the more beloved son who was born second. The Ramban counts this as a mitzvas aseh which prohibits transferring inheritance from the bechor to another son (see his additions to the Rambam's count at the end of Sefer haMitzvos, #12).
How to interpret the pasuk sheds light on a discussion in the Yerushalmi Baba Basra (8:4, 23b in Vilna ed.): R' Lo gave the bechor an equal portion as the other sons. R' Chagai asked: what of the pasuk "lo yuchal levakeir"? R' Elazar then responded, "yachol v'aino rashai".
The Pnei Moshe makes the assumption that everyone agrees that if the father (in his lifetime) makes a gift of his assets to a son other than the firstborn he can circumvent the prohibition of "lo yuchal levaker". R' Chagai only questioned what R' Lo did because he did not know the facts of the case. This reading is a bit difficult because R' Elazar's statement does not add anything that was not known at the start of the sugya.
The Tzion Yerushalayim, however, reads the the give and take of the sugya as debating how to understand the pasuk of "lo yuchal levaker". R' Chagai understood the pasuk to mean what it says: one cannot transfer assets; i.e. it is impossible to do. To which R' Elazar replied that "lo yuchal" does not mean it is impossible, but rather it means (as Rashi in P' Re'eh and the Ramban understood) that if done incorrectly there is a prohibition of making such a transfer.