Contractual Lien Clause
A lien is a security device.
A person in possession of another person's property may be retained by a person until such time as the debt has been paid.
That doesn't give a right to sell the property (see tort of conversion), but simply retain it pending payment of outstanding sums.
There are established categories of liens.
Outside these classes of relationship, there are no such rights recognised by law. The terms of a lien can however be established by contract: a contractual lien.
Example: Contractual Lien Clause
The contractor shall be entitled to retain possession and control of any materials supplied by the customer before or during the term until such time as the invoices of the supplier are paid in full.
Related: