The gemara states (רבא) that we learn out from בשגגה that אומר מותר does not go to גלות. The rishonim there ask as follows. The Gemara in Shabbos (68b) states that אומר מותר on shabbos is chayav a חטאת (except for certain extreme cases like תינוק שנשבע), meaning that אומר מותר is considered a שוגג. If so, why does he not go to גלות?
The rishonim give a number of answers:
1. Since by גלות it says שוגג many times it needs to be a higher level of שוגג , אומר מותר is either קרוב למזיד or קרוב לאונס and is not considered this level of שוגג and therefore there is no גלות. On shabbos however, the requirement for שוגג is less stringent and therefore אומר מותר is considered שוגג. Because גלות requires a higher level of שוגג, the gemara on 9a calls someone who intended to kill an animal and killed a person אומר מותר, however, this is a special din by גלות, by shabbos for example this would not be called אומר מותר
2. The Ritva and Ramban quote יש אומרים that the use of אומר מותר both on 7b as well as 9a is a different usage then in other places. In other words, the gemara uses the same phrase with a different meaning. They explain that the gemara on 7b when it says אומר מותר means the case that the gemara mentioned 9a ( someone who intended to kill an animal and killed a person).
There is a big נפקא מינה between the 2 answers. According to the second answer, if Reuven kills Shimon and says that he thought he was allowed to kill Shimon, it is a classic case of אומר מותר which is considered שוגג and he would go to גלות. According to the first answer this is exactly what the gemara excludes based on the pasuk.
The bottom line is that according to everyone, the usage of אומר מותר on 9a to mean someone who intended to kill an animal and killed a person, is restricted to גלות only, in other places (such as Shabbos) this would not be considered אומר מותר.