The Gemara discusses the following verse as proof that the security is owned by the lienholder:

Devarim 24:13

you must return the pledge to him at sundown, that he may sleep in his cloth and bless you; and it will be to your merit before the LORD your God.

הָשֵׁב֩ תָּשִׁ֨יב ל֤וֹ אֶֽת־הַעֲבוֹט֙ כְּבֹ֣א הַשֶּׁ֔מֶשׁ וְשָׁכַ֥ב בְּשַׂלְמָת֖וֹ וּבֵֽרֲכֶ֑ךָּ וּלְךָ֙ תִּהְיֶ֣ה צְדָקָ֔ה לִפְנֵ֖י יְהוָ֥ה אֱלֹהֶֽיךָ׃ 

Why does the verse consider it a “merit” if he doesn’t own the object? Ergo, the lienholder must have financial ownership based on the unpaid debt. Of course the pashut Pshat is that since he is holding the object as a Security, he may have a temptation to hold onto it and not return it to the debtor If he needs it to earn a living or to survive, such as clothing or a mill, and that is why the verse sees it as meritorious. It does actually have to mean that he owns it.

The mystics have a beautiful interpretation of this verse (see תקוני הזהר קמ״ד א:ב):

The security in question is our soul, entrusted to us by G-d. Not given to us, but merely handed over for safekeeping.  However, every night, as the Sun sets, our soul ascends back to heaven and is judged. This corresponds to the part of the verse that states you must return the pledge at sundown. The verse then questions if you do not have merit how are you be able to “lay down“, meaning, “How will you be able to go to the grave without merit?”, asks the verse rhetorically. However, if you do good deeds then when you lay down to sleep at night, G-d will see your merit and return your soul to you and give you a blessing for the next day. This corresponds to the part of the verse that says, “that he may sleep in his cloth and bless you; and it will be to your merit before the LORD your God.”

It is easy to lose sight that we do not fully belong to ourselves. We are not private property. We have obligations to our family, to our community, and even to our own soul to make sure it is taken care of. It goes without saying that something as dramatic as suicide is a total violation of this principle, but even small kinds of deaths like self-neglect are also a betrayal of the soul that has been entrusted to us by G-d. We simply don’t have a right to mistreat ourselves, let alone others.