I didn't hear the following saying until last year, so I don't know if it's common:
"Cut your coat according to your cloth"
It means to make the most of the limited money/resources you've got.
It kinda sums up the fixed budget/date and variable scope way of making Agile promises.
The sentiment also applies to how you adopt and adapt agile: don't moan about your situation being imperfect for agile (old ugly code? A disengaged customer? A dispersed team? Average staff?), do make the most of what you've got ... And then make it better.