Which informal logical fallacy is committed in the statement 'If you give him an inch, he will take a mile'?

The saying warns that a small concession will escalate to a large loss.

Giving an inch is projected to end in the taking of a mile.

One minor allowance is assumed to trigger a runaway chain of further demands.

No reason is offered for why the small step must lead to the extreme.

Predicting an extreme outcome from a slight first step is the slippery slope.