Would it be considered an oath (yameen) of immersion (ghamūs) if I were to swear to the traffic officer that the fire extinguisher is mine, knowing that I borrowed it with the intention of purchasing it later?

Lying is forbidden and is considered among the grave sins. Its sinfulness intensifies if an oath is sworn upon it, in which case it is called "al-Yamin al-Ghamus" (the diving oath), and it is also among the grave sins. The use of allusions (ma'areedh) is permissible if its purpose is legitimate. However, in this case, it is not legitimate because it involves circumvention of regulations. An oath is judged according to the intention of the one administering it if it involves the rights of others. Sincere repentance is obligatory for this false oath, and the Shafi'is have held that an expiation for an oath (kaffarat al-yamin) is due for it.