Blank cheque

cheque that has no numerical value entered, but is already signed

A blank cheque is a cheque where a person has filled all the details, including his signature, but has not written the amount that is to be cashed by the other person.

The term is a metaphor meaning "there's no limit". It may used to describe giving someone unrestricted funds for a project, for example "The city council gave the contractors a blank cheque to build the new library".