a worker's right to eat produce of his employer while harvesting

Last week we left off with a kashe – why is it that the Rosh paskens that tefisa does not work on a sfeika d’dina, but with respect to the question of whether a poel can eat only from the produce before him or from any produce (which the gemara leaves as a safeik) the Rosh paskens that tefisa does work?
The gemara (B.M. 92a) debates whether a worker “mi’shelo hu ochel”, eats the produce of the ba’al habayis as part of his salary, or “mi’shel shamayim hu ochel”, the Torah granted him permission to eat as a separate benefit above and beyond regular earnings - as Rashi explains, the right to eat is a form of gemilus chessed. The nafka minah is whether the worker can request that his share of the produce he could eat be given to his family – if it is part of his earnings, like salary, it is transferable to a third party, but if it is like tzedaka, then until the worker actually picks the produce he has no right to it and cannot transfer it to a third party.
The Kuntres haSefeikos writes that in a case of contested ownership, tefisa does not work where there is a sfeika d’dina because a halachic doubt is insufficient cause to challenge the ownership of the muchzak. However, the case of a worker eating the produce of the ba’al habayis is different. Here, “m’shel shamayim hu ochel” – the worker never claims a right to the produce based on ownership, but based on the tzedaka the Torah demands. In other words, the Torah allows a worker to eat food that is admittedly not his! The fact that the ba’al habayis is muchzak is irrelevant because the issue is not one of ownership, but an issur v’heter type question of whether the ba’al habayis is obligated in the tzedaka of feeding his worker in this context. In this case, the Rosh holds tefisa does indeed work.
Once we remove the halachos of poel from the domain of mamonos and place it in the issur v’heter domain, then we open the door to other questions. For example, the Mishna (Ma’asros 2:7) indicates that it is possible to make a tnai that one’s son can eat on one’s behalf. The Minchas Chinuch asks why this is not a case of “masneh al mah shekasuv batorah”, creating a tnai that violates the Torah’s law – even R’ Meir who allows such a tnai in dinei mamonos would not allow it in the domain of issur v’heter.
More to come bli neder, as R’ Chaim as a completely different understanding of the sugya.