In discussing this question Kant first mentions the apparent paradox involved in the very idea of a permissive law. Laws generally involve practical necessity whilst the notion of a permission seems, by contrast, to involve practical contingency. So a permissive law appears contradictory as implying a necessity for some contingency. However Kant seeks to escape this consequence by speaking of the different ways "the object of the law" can be meant (Ak. 8: 347n). The prohibition involved in the law here is directed to the future. There can be future permission to acquire a right in the manner forbidden by the second preliminary article. By contrast, the permission is directed to the present state of possession. The reason for the present permission concerns the general transition from the state of nature to the civil condition. In the process of this transition occurring there is a possession held which, whilst not in accord with right, is still, as Kant puts it, "possession in good faith".
This "possession in good faith" is said to accord, not with right, but with a permissive law of natural right. This type of possession is still an odd one since Kant makes clear that once its cognition has occurred it would not be granted either in the state of nature or in the subsequent civil condition so its right to be granted is only in terms of transition between two states that would, in themselves, grant no ground to it. The fact that there is a transition between the state of nature and the civil condition in which there are in effect permissive laws that neither of the states in question would grant indicates a peculiarity about this intermediate state. Rather than seeking to justify this concept further Kant merely indicates that the notion of a permissive law has to be specified in the way given since the only other way is to state a law of prohibition and then build in exceptions to the law into the formula of the law itself, something he thinks would ensure that there were no universal laws but only general ones.
The other interesting point worth raising is why it is that Kant's discussion of permissive law contains no reference to the 3rd and 4th preliminary articles (concerning standing armies and the national debt) but only the 2nd (on inheritance). Given that the 3rd and 4th preliminary articles have been treated in the same permissive way as the 2nd one would have expected the reasoning for the idea of a permissive law to include some consideration of all of them and not merely one of the articles effected.