Hi,
read the PS documentation. Not just for object C_PRPS_USR. The creators of PS anticipated that different customers will require some custom fields on project/WBS structure. Hence they added custom fields USR00 - USR11 that can be used by customers. They are not used by SAP and they have different types. There are some character based, some date based and so on. The text that is displayed is controlled in configuration. In your case somebody decided that USR04 and USR07 will be used for budget. USR04 is meant for quantity and USR07 is meant for value. I am not sure why standard budgeting solution from SAP wasn't used but that's not important. These fields can be defined in SPRO -> Project System -> Structures -> Operative Structures -> User Interface Settings -> Create User-Defined Fields for WBS Elements. The fields can be grouped by field key. For every field key you have an option to assign authorization object that will be checked. The object must have exactly same structure as object C_PRPS_USR. Hence you need to create a new custom object as a copy of C_PRPS_USR, assign it in SPRO to your fields. After that you can run authorization trace and see what is being checked by SAP.
Cheers