Boron halides react with molecules having nonbonding pairs as shown below. Although the product has no overall charge, there are formal charges at boron and at nitrogen as a result of formation of the bond.
 

 

Boron can be considered to "own" four bonding electrons (one more than it has as a nonbonded atom), and hence it has a negative formal charge.

Nitrogen can be considered to "own" one fewer electrons as its nonbonding pair has become bonding and must be shared with boron. The total number of electrons that nitrogen "owns" is four (one for each bond). As elemental nitrogen has 5 valence electrons. Nitrogen in this Lewis structure has a positive formal charge.