The Shulchan Aruch (H”M, 418:13) rules like the Chachamim (Bava Kamma 62), that one who burns a fire is not liable for embedded items (tamun). As the braisa details, this is only true if the fire was lit in one’s own property, and then spread to someone else’s. A fire lit in someone else’s property obligates the burner in the damages for embedded objects. This is due to the fact that the verse which excludes tamun is in the basic case of a fire described in the Torah – when a fire exited one’s property, and then damaged. However, liability for tamun in the case of a fire lit elsewhere is only for items normally embedded – and for which the burner should have considered may be burned. Therefore, in a field, he is only liable for farm implements, whereas in a house, he is liable for all items.
The Gemora earlier in B”K (22-23), in the topic of whether a fire causes liability as the burner’s arrows or property, discussed the case of tamun. The Gemora stated that according to Rabbi Yochanan (whose opinion is the halachah), who holds that a fire is like the burner’s arrows, there seems to be no reason to exclude tamun: if a person shot an arrow, he’s liable for any damages, even on embedded items. Therefore, the Gemora states that Rabbi Yochanan only excludes liability for tamun when kalu lo chitzav – the direct effect of the fire has stopped (e.g., by being blunted by a wall, which then collapsed). In that case, the liability is only for the property aspect of the fire, since he should have put up a firewall to stop it from spreading. Even though Rabbi Yochanan holds that a fire causes liability as an arrow of the burner, Rabbi Yochanan agrees that a fire can also cause liability as the burner’s property, and would be considered so if the burner could have stopped the fire and didn’t. Therefore, the Shulchan Aruch rules that the exclusion of tamun only applies to a fire which was blunted by a wall.
The Gr”a (418:33), however, claims that this understanding of the Gemora on 23 is incorrect, and is not the way the Rambam and the Rif read it. Instead, the Gr”a says that when the Gemora clarified that Rabbi Yochanan agrees to a property aspect of a fire, the Gemora was reversing the limitation of tamun to kalu lo chitzav. The Gemora’s original question was how a person could be exempt for paying for tamun damages. The Gemora here states that a burner is liable for tamun when he lights the fire in the grain owner’s property. However, just as when someone fires an arrow, wherever it goes is considered his action, when someone lights a fire anywhere, wherever it goes should be considered his lighting, and therefore, all cases of tamun should be liable. The Gemora on 23 first answered that when the wall blunted the fire, it’s not an arrow anymore. However, the Gemora then reverses this, and states that a fire is different than an arrow, because a fire can be interrupted in mid flight. Therefore, the verse’s exclusion of tamun is only when the fault of the burner was simply not stopping the fire. The Gr”a therefore disputes the exception that the Shulchan Aruch places on the limitation of tamun, and instead holds that the burner of a fire is always not liable for tamun, as long as he lit the fire in his own property, and it spread elsewhere. [See Gra on B”K 23, note 1 for a different reading of the Gemora there.]
The Shitah quotes Rabbi Yehonosan who points out that we must explain that the case of a barley pile coated in wheat is a case where the field owner only allowed barley. If this would not be the case, there would be no reason to exempt him from paying for the wheat covering, which he allowed and saw. If so, the need for this case – even though we already learned the case of bringing a full wheat pile when only allowed to bring in barley – is to teach us that if the owner only allowed barley, he is not even obligated in the small difference that a wheat covering adds.