In Casto v. Casto, 508 So.2d 330 (Fla. 1987), the Court considered the elements upon which a trial court could rely to set aside a postnuptial agreement. In doing so, we delineated the following parameters: First, a spouse may set aside or modify an agreement by establishing that it was reached under fraud, deceit, duress, coercion, misrepresentation, or overreaching. The second ground to vacate a settlement agreement contains multiple elements. Initially, the challenging spouse must establish that the agreement makes an unfair or unreasonable provision for that spouse, given the circumstances of the parties. . . . Once the claiming spouse establishes that the agreement is unreasonable, a presumption arises that there was either concealment by the defending spouse or a presumed lack of knowledge by the challenging spouse of the defending spouse's finances at the time the agreement was reached. The burden then shifts to the defending spouse, who may rebut these presumptions by showing that there was either (a) a full, frank disclosure to the challenging spouse by the defending spouse before the signing of the agreement relative to the value of all the marital property and the income of the parties, or (b) a general and approximate knowledge by the challenging spouse of the character and extent of the marital property sufficient to obtain a value by reasonable means, as well as a general knowledge of the income of the parties. The test in this regard is the adequacy of the challenging spouse's knowledge at the time of the agreement and whether the challenging spouse is prejudiced by the lack of information. (Casto, 508 So.2d at 333.) Thus, Casto dictates that a postnuptial agreement may be set aside where there is a direct showing of fraud or overreaching by one party or, assuming the agreement was unreasonable or unfair, where the challenging party shows by unrebutted presumption that he or she did not have adequate knowledge of the marital property at the time the agreement was entered into, or that the other party concealed assets. The Casto court qualified its reasoning by noting: "As reflected by the above principles, the fact that one party to the agreement apparently made a bad bargain is not a sufficient ground, by itself, to vacate or modify a settlement agreement. . . . A bad fiscal bargain that appears unreasonable can be knowledgeably entered into for reasons other than insufficient knowledge of assets and income. There may be a desire to leave the marriage for reasons unrelated to the parties' fiscal position. If an agreement that is unreasonable is freely entered into, it is enforceable." (Id. at 334.)