On an instrument, a mere reference to another writing-"As per contract"- does not render the instrument nonnegotiable.

Nonnegotiable

Nonnegotiable describes an item that cannot be transferred or endorsed to another party, often because it is final and not open for discussion or modification.

Reference

An act of mentioning or alluding to something or someone, often used for validation, citation, or acknowledgment in academic or professional contexts.

Instrument

is a legal document formally executed and capable of creating rights and obligations, such as contracts, wills, or checks.