give (someone) an inch and (they'll) take a mile

Translations
дай (кому-то) палец, (он) всю руку откусит
If you give him an inch, he'll take a mile.
Если ты дашь ему палец, он всю руку откусит.
Definitions
This phrase means that if you give someone a small amount of freedom or leniency, they may take advantage of it and demand much more than what was originally offered.