What is the idiom, expression or proverb for 'If you let them use you once they will use you for life'?

What is the idiom, expression or proverb for

If you bend once, they will bend you for life.

In Indian culture in marathi language, we have a saying

"Jithe oli/mau mathi, tithe atti" which literally means "wherever wet/soft ground, there is exploitation." The meaning is wherever people find niceness, kindness, or generosity, people will take advantage of their niceness. That is, if one yields or allows himself to be used then the other party squeezed them to the last drop.

This is usually used in business, like client vs. subcontractor relationship or client vs. service provider relationship or client vs. customer relationship, or in an employer/employee context.

Two phrases that are close are:

Give them an inch and they will take a mile.

e.g. John would not give an inch in the negotiations with the opposition.
to open the floodgates

e.g. The company agreeing to the workers' demands for better pay would open the floodgates to demands by workers in other departments.

There is a popular children's book called If You Give a Mouse a Cookie that describes how giving in to a greedy party's demands will just result in a cascade of demands -

If you give a mouse a cookie, he will ask for a glass of milk.

There is a whole series of books based on this premise (e.g. If You Give a Pig a Pancake), but the first is familiar enough (it was a New York Times Best Seller) that simply saying if you give a mouse a cookie will often elicit an understanding of the unspoken follow-up, meaning that if you cave to unreasonable wishes, your kindness will be met with exploitation.

The camel's nose is a metaphor that is sometimes used for this. It is supposedly of Arab origin, but was adopted into English around the mid-19th century, and may in fact be British in origin.

An early example is a fable printed in 1858 in which an Arab miller allows a camel to stick its nose into his bedroom, then other parts of its body, until the camel is entirely inside and refuses to leave. Lydia Sigourney wrote another version, a widely reprinted poem for children, in which the camel enters a shop because the workman does not forbid it at any stage.

This is sometimes pithily stated as "If the camel once gets his nose in the tent, his body will soon follow," or "Beware of the camel's nose."

It's worth noting that this turn of phrase is not as common as some of the less "colorful" options provided in this thread, so it may require some explanation if used.

A literary reference would be “Dane-geld” by Rudyard Kipling: