We are warned not only in civil law, but also in the holy Scriptures, to avoid fraud in every agreement, as is clear from the example of Joshua and the Gibeonites.
66. In everything, therefore, good faith is seemly, justice is pleasing, due measure in equity is delightful. But what shall I say about contracts, and especially about the sale of land, or agreements, or covenants? Are there not rules just for the purpose of shutting out all false deceit, and to make him whose deceit is found out liable to double punishment? Everywhere, then, does regard for what is virtuous take the lead; it shuts out deceit, it expels fraud. Wherefore the prophet David has rightly stated his judgment in general, saying: “He has done no evil to his neighbour.” Fraud, then, ought to be wanting not only in contracts, in which the defects of those things which are for sale are ordered to be recorded (which contracts, unless the vendor has mentioned the defects, are rendered void by an action for fraud, although he has conveyed them fully to the purchaser), but it ought also to be absent in all else. Candour must be shown, the truth must be made known.
67. The divine Scriptures have plainly stated (not indeed a legal rule of the lawyers but) the ancient judgment of the patriarchs on deceit, in that book of the Old Testament which is ascribed to Joshua the son of Nun. When the report had gone forth among the various peoples that the sea was dried up at the crossing of the Hebrews; that water had flowed from the rock; that food was supplied daily from heaven in quantities large enough for so many thousands of the people; that the walls of Jericho had fallen at the sound of the holy trumpets, being overthrown by the noise of the shouts of the people; also, that the king of Ai was conquered and had been hung on a tree until the evening; then the Gibeonites, fearing his strong hand, came with guile, pretending that they were from a land very far away, and by travelling so long had rent their shoes and worn out their clothing, of which they showed proofs that it was growing old. They said, too, that their reason for undergoing so much labour was their desire to obtain peace and to form friendship with the Hebrews, and began to ask Joshua to form an alliance with them. And he, being as yet ignorant of localities, and not knowing anything of the inhabitants, did not see through their deceit, nor did he enquire of God, but readily believed them.
68. So sacred was one's plighted word held in those days that no one would believe that others could try to deceive. Who could find fault with the saints in this, namely, that they should consider others to have the same feelings as themselves, and suppose no one would lie because truth was their own companion? They know not what deceit is, they gladly believe of others what they themselves are, while they cannot suspect others to be what they themselves are not. Hence Solomon says: “An innocent man believes every word.” We must not blame his readiness to believe, but should rather praise his goodness. To know nothing of anything that may injure another, this is to be innocent. And although he is cheated by another, still he thinks well of all, for he thinks there is good faith in all.
69. Induced, therefore, by such considerations to believe them, he made an agreement, he gave them peace, and formed a union with them. But when he came to their country and the deceit was found out—for though they lived quite close they pretended to be strangers—the people of our fathers began to be angry at having been deceived. Joshua, however, thought the peace they had made could not be broken (for it had been confirmed by an oath), for fear that, in punishing the treachery of others, he should be breaking his own pledge. He made them pay the penalty, however, by forcing them to undertake the lowest kind of work. The judgment was mild indeed, but it was a lasting one, for in their duties there abides the punishment of their ancient cunning, handed down to this day in their hereditary service.
Source: On the Duties of the Clergy (New Advent)