Regarding a bird that a metzora must set free Rava says that it must be permissible to be eaten, for, if not, although a finder would be permitted to eat it by means of rov, it is nevertheless a forbidden bird. The Torah would not command us to send out a forbidden bird even if it will become permitted. We see that even if it will be permitted, it is considered a takala – stumbling block (the Gemara’s terminology).
The Steipler says that the same can be said for the Rambam’s opinion, namely that a safek mi’de’oraisa is permitted mi’de’oraisa. While it is permitted mi’de’oraisa, its actual status is unchanged. Thus an animal that is a safek bechor may not be entered into a corral for ma’aser purposes – even according to the Rambam.