The $-$bromination of carbonyl compounds by $B{r}_2$ in acetic acid is limited to aldehydes and ketones because acids, este and amides don't enolize to a sufficient extent. Carboxylic acids, however, can be $-$brominated by first converting the carboxylic acid to an acid bromide by treatment with $PB{r}_3.$ Following enolization of the acid bromide, $B{r}_2$ reacts in an $$ substitution reaction. Hydrolysis of the acid bromide completes the reaction.

Draw curved arrows to show the movement of electrons in this step of the mechanism.