PJB - Contract Line -> Billing control section -> Billing Resource field is grayed out

Summary:

Content (required):

We have a contract with 3 lines and For one of the line, we need to remove the Billing resource from the contract line billing control section. But, when we do amendment of the contract, Contract Line - Billing control section - Billing Resource field is grayed out and not allowing any changes to Billing resource value.

No Invoice or revenue has been generated on this contract line.

Any insight into this is really appreciated.

Version (include the version you are using, if applicable):

23B

Code Snippet (add any code snippets that support your topic, if applicable):