When I have to control a field, I set up a column pre-validate script with an IsMember check, then SetError when I want to block changes.  Permissions are controlled not by Role check-boxes, but by whether the user has your new custom role.

retVal = oSession.AsObject(oSession.Security).IsMember("rolename")

Will return a value > 0 if user belongs to specified security role or 0 if not a member.

Here is a script that was distributed by the renown Elliot at the Sage Summit.  Only members that are members of customization group Admin can change the credit limit.  You can adapt to any field 

if oSession.ASObject(oSession.Security).IsMember("admin") <> 1 then
retval = oScript.SetError("Sorry you are not authorized to change the credit limit. Nice try!")
end if

Thanks guys, I just can't see where the variable is for credit limit in that example, if it's there at all - it seems like it isn't and I don't know the syntax or object names to be able to adapt this script.

Or maybe this needs to be added to the salesperson object somehow so it wouldn't need anything else added to it - I've never used scripts in Sage. I'm not sure where I'd begin with this. 

The script BigLouie gave is not coded to any specific field, which is very flexible.  You would associate it with a table / field, when you add the event trigger.

I copied/pasted to script Louie shared into a new script I added in UDS Maintenance - I only changed the role from "admin" to a new role so I can can apply this role to people who shouldn't have the admin role, but need this function. I didn't add any rights in this role - hopefully it'll still work... 

I entered UDF and Table Maintenance and went into AR Customer Master (is this the correct Table?)

I set Event to "Column - Pre-Validate" as shown in Kevin's example (is this correct for my purposes as well?)

I selected SalespersonNo in Field (is this correct for my purposes? Do I need to do this entire process for all of the other Salesperson fields that are listed?)

I selected the script I added earlier in UDS Maintenance (I'm hoping at least that part is correct)

Anything needed for Priority? Default is 500. 

I'm hoping this works and doesn't explode and catch fire or dud either.

I simply clicked OK after doing all of this in the window below. 

I have an admin account and although I didn't apply the new role I created (SalesPerson) to myself, I was still able to change the Salesperson in Customer Maintenance. I'll test with another account that has less rights.

Didn't work - still let me change Salesperson with a user account that shouldn't have that ability. Can someone check that I used all the right Table, Field and Event settings? Do I need to create additional scripts through this same process but modified in various ways to get this to work?

Company Maintenance - Allow External Access.

Compile the script (there is a prompt after you add the script event, and close the screen).

Restart your Customer Maintenance screen after compiling.

In User Maintenance you need to enter Admin in Customization Group field.

It is reading Admin from Customization Group in User Maintenance. 

Sorry BigLouie.  Customization Group only applies to panel customizations in Customizer (instead of All or user specific panels). 

The IsMember function looks for a named role applied through User Maintenance.