The letter you refer to is a legal agreement called a Descargo Laboral. There is specific wording which you need which confirms that the person agrees that they waive all rights to any claims against you for any money outstanding now or in the future. It should be witnessed. If they don't sign that agreement, then they can still go to court, and I have known this to happen - people get their prestaciones paid, agreed with the secretaria, and then go to court as they haven't signed a Descargo.
With respect to your bitch about the toxic environment in my company, you don't know what the company is or does, you don't know how many staff I employ, you don't know how many years I have been doing what I'm doing, nor any idea of my background or employment history. So clearly it is foolish to suggest that because I have experience in dismissals there is a toxic environment.