Cassian's Conferences are a compendium, in dialogue form, of the wisdom of the Desert Fathers on how we train our souls and perfect our hearts. It is one of the great monastic classics, and with his Institutes is very influential for moral theology (to name just the most widely known example, the first rudiments of what would eventually become the famous list of the Seven Deadly Sins begins with Cassian). The discussion of lying occurs in the seventeenth Conference, and occurs because of a matter of some dramatic interest. The monks involved in the dialogue had promised the Elders at their monastery that they when they toured the desert of Skete (where the most famous ascetic hermits and monks were), gathering spiritual knowledge, that they would do so quickly and return to their monastery at once. When they did so, however, they realized that this promise was a heavy burden: by staying with the Desert Fathers, the monks could grow in their spiritual and moral life, and, indeed, there was too much spiritual wisdom among the Desert Fathers to make their tour any more than a grazing of its surface; but they were bound by their promise to return to the monastery as quickly as possible.
In light of this, Cassian (as one of the characters in the dialogue) suggests that they consult the Desert Father with whom they are staying, Abbas Joseph, and treat his decision on the matter as if it came from God himself. And so they agreed. When they meet again for prayers, Joseph notices that they are all a bit downcast and asks the reason. They tell him, and he agrees that keeping one's oaths and promises is very important, and that a monk ought therefore never to promise anything hastily. But as they have already promised, it does pose a dilemma. Joseph's solution is that they should break the promise, because
the will in choosing should incline to that side which involves a loss that is more tolerable, or can be more easily made up for by the remedy of making amends. If then you think that you will get more good for your spirit by staying here than what accrued to you from your life in that monastery, and that the terms of your promise cannot be fulfilled without the loss of great good, it is better for you to undergo the loss from a falsehood and an unfulfilled promise (as it is done once for all, and need not any longer be repeated or be the cause of other sins) than for you to incur that loss, through which you say that your state of life would become colder, and which would affect you with a daily and unceasing injury.
He then goes on to discuss why some promises should be broken, and answers some of the monks' worries. Then we come to the key text for our purposes: Germanus, the monk who has primarily been presenting their worries to Joseph, says that they are worried that breaking their promises could serve as a bad model for other younger monks, in whom it might encourage lying. Joseph suggests that whether it is so or not will be more determined by the state of the souls of the younger monks rather than from their action, but he also goes on to argue that some lying is salutary.
It is important to be very careful here. It is clear from the examples and arguments used by Joseph that at least three things are included under the term 'lying' that later tradition distinguishes: promise-breaking, saying something false in order to mislead, and 'hiding' (i.e., presenting a different face to the world than one has in private, as when a monk hides the fact that he is fasting). Indeed, one of the reasons Augustine plays such a big role in later moral theology about lying is that he is one of the first to recognize that the term was used so ambiguously and to begin roughing out a scheme for distinguishing these different uses; and most of the history of the moral theology of lying has been a series of refinements on Augustine's basic refinements. It becomes generally recognized in the moral theology that follows Augustine that some promises should be broken and some 'hiding' is permissible, and that these aren't even necessarily lies at all; so we need to be careful in how we compare what Cassian says to what Augustine or someone in the Augustinian tradition like Aquinas says -- the two are separated by the fact that Cassian is using a very loose colloquial term from common conversation about morality and Augustine, worried about some of the confusions and sloppiness of this sort of colloquial term, has put in some work to make it more precise.
One thing that we must get out of the way, however, is eliminate any suggestion that Joseph argues that any kind of lie is simply OK considered as a lie. His argument is very different: it is that a lie is like hellebore, which will poison a healthy person but in extreme cases can be used as a medicine to cure him. Joseph insists quite clearly that only in cases of utmost necessity should one even consider a lie like Rahab's: only because on the spur of the moment there was no other obvious way to save the spies was Rahab's lie something that should have been done, and it is only because of that that we should not condemn the action. He also talks about a lie (at least sometimes) involving a stain and about its interfering with one's own spiritual perfection. Indeed, the sort of necessity involved can only be necessity arising where there is love of neighbor and no complete danger of our own salvation; as he will say later, everything should be done for "the preservation of love, for which all things else should be disregarded." Given a forced choice between lying (to our spiritual detriment) and failing to love our neighbor, there is no real contest: we are to love our neighbor in the best way we can, as long as in doing so we aren't putting our very salvation in danger. But it's important to keep in mind, throughout, that while some of the things Cassian calls 'lying' are actually lies, what he means by 'lying' is just the various ways in which we can knowingly mislead people. (Actually, it's even broader than what we would usually mean by this, since it includes among other things breaking promises you really intended to keep when you made them.) And pretty much everyone in the tradition of the Church is agreed that there are ways in which we knowingly mislead people that are not to be condemned in any way.
When you peel away all these things that Cassian calls 'lying' that the Augustinian tradition does not, and just focus on the things that everyone agrees are lies, then the two traditions don't actually seem very far apart. Joseph repeatedly talks about lying in terms that make it sound like its a venial sin; some of the things he says suggests that he might think that there can be special dispensations (as Scotus allows), and possibly he thinks there have been (apparently unlike Scotus). What we really find with Cassian is moral theology in a transitional state: his precise view is unclear because he has no precise view (it doesn't entirely help that this is a dialogue and thus that the answers are built dialectically and dynamically). This is not to say his view is not well-developed: with Cassian we find moral theology beginning to take into account a massive amount of moral examples and advice that have to do with lying and begin to make sense of it all at once rather than in a piecemeal way. He lacks, however, Augustine's somewhat closer analysis of the different ways in which we might knowingly mislead someone, and because of this he makes no distinction between breaking a promise you shouldn't have made, saying something false for a good reason, and not being completely open about everything. Given how broadly he is taking the term, his basic position is actually something everyone agrees with: there are times when it is necessary as a matter of charity to try to mislead someone, knowing that they will likely be misled. He does apply this at times to lies in the narrow sense but does not do so in such a way as to suggest that the lie is therefore not defective; he does say that sometimes such lies are blameless, when charity is on the line, but does not clarify the sense in which this is so. And thus it is not possible to put Cassian in clear opposition to Augustine on the issue of lying; he says things that can be taken as either consistent or inconsistent with Augustine depending on how one takes his claims to map onto precisions he does not make. It is even possible to read him as, with perhaps a minor exception or two, taking a much stronger stand against deception generally than many in the Augustinian tradition do.
Regardless, it's clear from Cassian's discussion that any kind of deception is dangerous business, and never to be undertaken lightly.