Yes they could still sue you if you pay, but your defense would be that an oral agreement existed to modify the payment terms of the note you signed with them. Remember that since your father is a co-signer, they will come after him if you do not pay.
You have two options: pay the $100 per month or try and negotiate a lump sum payment for something less than the full amount they claim is owed. I recommend the latter option if at all possible. Good Luck.