It depends

Assuming the dispute is not liquidated, that is, you or the other guy honestly beleived that there was a dispute on how much money you owe or are owed, then your offer to pay less, in order to dismiss the lawsuit, is an accord. If the other accepts, his acceptance is a satisfaction so that your original obligation is discharged and he has no legal basis to sue again on the original dispute. To do that, you must on the check, write unequivocally that the money is for discharging the original dispute.