Can 2nd lien holder file suit against me, they issued a full deed of release on a heloc to facilitate a DIL?

Credit union in Missouri issued a full deed of release to the 1st lien holder to facilitate a mortgage deed in lieu (DIL) transaction in exchange for $2,000. Total debt was $5,900. My credit report says the credit union charged of the $3,900 difference. Now a law office saying they represent the credit union says they will file suit if I don't come up with the $3,900 difference. The full deed of release says "in consideration of the FULL PAYMENT of said debt does hereby acknowledge satisfaction of said deed of trust and release the property therein described from the lien and said affects of the same" Am I legally responsible to pay this? Who is likely to win if this goes to court? Your insights and experience is appreciated. Thanks