After borrowing $$500$ from her roommate to cover rent, Shayla writes "I acknowledge that I owe Sabrina $$500,$ payable out of the proceeds of the sale of my car, a $2008$ Saturn Ion, which I promise to advertise for sale next week. Payment is to be made on or before three months from today.

What type$($s$)$ of instrument is this? Is it negotiable? Explain why it is negotiable or why it is not negotiable.