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 illicit an understanding of the unspoken follow-up, meaning that if you cave to unreasonable wishes, your kindness will be met with exploitation.

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.