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What's an idiom for the idea that if you do something good for someone you will get the favour back, like a reciprocal favour?
An edit after several answers were already given follows.
For example, in French there is an idiom renvoyer l'ascenseur as in if you do someone a favour he will return the elevator to you
"Good", as opposed to negative implications like you reap what you sow and those who live by the sword die by the sword etc
An idiom for this would be, as Jim suggested, "You rub my back, I'll rub yours," otherwise stated as "You scratch my back, I'll scratch yours."
Another excellent term for this, as mentioned by Lambie, would be "Quid pro Quo", which means "something given or received for something else." I wouldn't call this an idiom though.
A google search for "Quid pro Quo" brings up another relevant term: "One hand washes the other," meaning mutual favors are exchanged.
I think you need one good turn deserves another.
One good turn deserves another: If someone does you a favour, you should take the chance to repay it — Lexico
If you do something good for someone, they'll do you a favour in the same way (reciprocate). It is usually used in a positive way, to support each other in small acts of kindness, but can be used to talk about corruption. I've also heard it used in a crime TV show where one criminal helps another to commit a crme and says one good turn deserves another.
A recent quote from TimeOutIn:
Maybe more people will start to believe that one good turn deserves another, that lending your hand to those in need can benefit yourself later.
There are some other expression like tit for tat and hoist by one's own petard but they have negative connotations.
Note that I provided this answer before the question was edited, which has now served to partially invalidate it.
From "Karma: What Goes Around Comes Around" by Pandit Dasa:
“What goes around comes around” or “as you sow, so shall you reap” is the basic understanding of how karma, the law of cause and effect, works. The word karma literally means “activity.” Karma can be divided up into a few simple categories — good, bad, individual and collective. Depending on one’s actions, one will reap the fruits of those actions. The fruits may be sweet or sour, depending on the nature of the actions performed. Fruits can also be reaped in a collective manner if a group of people together perform a certain activity or activities.
In short, whatever you put out to the universe and others around you is what you can expect to get back in return.
According to the theory (and the idioms), if you do bad things, then bad things will happen to you; but if you do good things, then good things will happen to you.
What is often referred to as the golden rule, is a rephrasing of what @Micah has already offered.
Merriam-Webster phrases it thusly: the Golden Rule, usually capitalized G&R : a rule of ethical conduct referring to Matthew 7:12 and Luke 6:31.
"Do to others as you would have them do to you."
Though I have most often heard or read this with a slight modification which makes the phrasing sound more archaic and "biblical."
"Do unto others as you would have them do unto you."
This formulation does not precisely cover the meaning you are going for, as the saying does not contend that doing good deeds will in fact result in reciprocation. But it does offer an optimistic view that abiding by this principal is the best means for eliciting gestures in kind.
