give one an inch, and he will take a mile
If you give some people a little or yield anything, they will want more and more; some people are never satisfied.
I gave Billy a bite of candy and he wanted more and more. If you give him an inch, he'll take a mile.
The counselor said to Jack, "No, I can't let you get a haircut until Saturday. It's against the rules, and if I give an inch, someone will take a mile."
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Source: A Dictionary of American Idioms