What is an idiomatic way of saying that there is no reason to be honest just for the sake of being honest. That is, if you can benefit from a dishonest action and know that you definitely won't suffer any financial, reputational, or other damage that outweighs the benefit, then you should act dishonestly. To put it simply, if you can cheat and get away with it, then cheat.
Is there a common/idiomatic expression for this in English?
As stated in comments, phrases that encourage dishonesty or other forms of underhandedness are less direct in English than more-upstanding phrases like "Love thy neighbor," but I've thought of a few options that can carry the same implications as the Chinese phrase you mentioned, especially given the right context.
This phrase is actually quite common, meant to convey that playing by the rules (whether formal or informal) leads to poor outcomes, and that those who are willing to bend or break moral or other conventions, exploit loopholes, etc. often come out ahead. There's even an eponymous song by popular punk band Green Day from an album that sold 200,000 copies. Wikipedia has more on this general idea.
This phrase is a more colorful version of "life's not fair." In some cases, this phrase is used to describe injustice, but it is just as often used to tell someone that they should take care of themselves, because no one else will. With the right context, this may simply mean "be cautious," but it can also mean "if you can cheat, then cheat." See the culture section of this page or perform a web-search for this phrase.
A similar version of this phrase "it's not illegal unless you get caught" was already mentioned in comments. For non-fictional examples of this mantra, see the "Real Life" section for this idea on TV Tropes.
This phrase is sometimes used to describe someone's tactics e.g. "She was taking no prisoners," or "He had a take-no-prisoners attitude" but it may also be given as advice/encouragement to be relentless. Depending on context, you will also hear "give 'em hell!" (see bottom definition on this page) though usually this is applied to a specific situation, whereas "take no prisoners" may be stated by itself.
This phrase has many variants including "never pull your punches", "don't pull any punches", and "pull no punches." It is similar to "take no prisoners" in that it can be either descriptive or prescriptive, especially in adversarial situations. Other less-catchy forms of this including "Take every advantage you can get" are more general to situations that are tough, but not necessarily zero-sum winner-loser arrangements.
Additional phrases:
Concepts:
Never give a sucker an even break.
This phrase means that one should take advantage of those who are gullible or not well informed, if given the chance.
I've heard "It's only against the law[/rules] if you get caught" used fairly often, and while it doesn't encourage one to engage in such behavior where all other things are equal, it does have a connotation similar to what you're after.
What about all's fair in love and war? That is, things that might normally be considered unacceptable are/can be used.
From Merriam-Webster:
—used to describe a situation in which people do not follow the usual rules of behavior and do things that are normally considered unfair
Sure, it was underhanded to steal his customers, but all's fair in love and war.
A discussion of the phrase's origins can be found here.
"If you aren't cheating, you aren't trying." is a fairly common phrase, at least in the U.S.
The end justifies the means
From The Free Dictionary. Basically indicates that as long as the objective is met, how you get there becomes immaterial. I should mention that this does carry a slightly heavier ethical questionability.
My husband's favorite: "No blood - no foul." Meaning: if there is no evidence that you did something wrong - then you didn't do anything wrong.
There's a fool walks across London Bridge every minute. He's your customer.
This was a common saying, so I am informed, in post war Britain. I cannot find it referenced but the person who I first heard had used the saying was one of Lord Rank's right hand men in the 1950s.
... there is no reason to be honest just for the sake of being honest.
This sentiment could be implied by expressing a dismissive attitude towards legality.
Illegal is just a sick bird.¹
I'm unsure of the origin but it's been around for several decades at a minimum.
¹ Not sure if it really needs explanation but the sarcastic homonym of illegal and ill eagle provides the implied disdain towards unwavering lawfulness
As far as acquisitions are concerned, we have Collis Potter Huntington —
Whatever is not nailed down is mine. What I can pry loose is not nailed down.
The following two excerpts are from Wikipedia: Collis Potter Huntington
Huntington, from his base in Washington, was a lobbyist for the Central Pacific and the Southern Pacific in the 1870s and 1880s. The Big Four had built a powerful political machine, that he had a large role in running. He was generous in providing bribes to politicians and Congressmen. Revelation of his misdeeds in 1883 made him one of the most hated railroad men in the country.
Huntington defended himself:
The motives back of my actions have been honest ones and results have redounded far more to the benefit of California than they have to my own.
And
Huntington described his activities in a series of private letters to David D. Colton, a senior financial official of his railroads. After Colton's death, litigation opened his files in 1883 and Huntington's letters proved a huge embarrassment, with their detailed descriptions of lobbying, payoffs, and bribes to government officials. They showed Huntington to be an active, profane, and cynical promoter of his companies and display his eagerness to use money to bribe congressmen. The letters did not demonstrate that any cash actually changed hands with any official, but they revealed the tenor of Huntington's morals.[12]
His biographer says:
he was vindictive, sometimes untruthful, interested in comparatively few things outside of business, and disposed to resist the idea that his railroad enterprises were to any degree burdened with public obligations. There is, on the other hand, no question with respect to his indomitable energy, his shrewdness in negotiation, his independence of thought and raciness of expression, and his grasp of large business problems. He was the dominant spirit among the small group of men who built up the Southern Pacific system, and that great organization remains his monument.
