Hi,

you can use onChange + GlideAjax

onChange catalog client script on that variable "Select RITM"

UI Type - ALL

Applies on Catalog Item - True

function onChange(control, oldValue, newValue, isLoading) {
	if (isLoading || newValue == '') {
		return;
	}


	if(oldValue != newValue){

		var ga = new GlideAjax('checkRecords');
		ga.addParam('sysparm_name', "checkRecordPresent");
		ga.addParam('sysparm_ritm', newValue); // give here user_id variable name
		ga.getXMLAnswer(function(answer){
			if(answer != ''){
				alert("Billing Rate Value is " + answer);
			}
		});
		//Type appropriate comment here, and begin script below
	}
}

Script Include: It should be client callable

var checkRecords = Class.create();
checkRecords.prototype = Object.extendsObject(AbstractAjaxProcessor, {

	checkRecordPresent: function(){

		var id = this.getParameter('sysparm_ritm');			

		var gr = new GlideRecord('sc_req_item');
		gr.addQuery('sys_id', id);
		gr.query();
		if(gr.next()){
			return gr.variables.billingRateVariableName;  // use valid variable name here
		}
	},

	type: 'checkRecords'
});

Regards
Ankur