functions.php

<?php
// https://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters
function xyz_custom_orderby( $query_args ) {
	// Use meta_value_num for numeric sorting (if issues with meta_value)
	$query_args[ 'orderby' ] = 'meta_value';
	$query_args[ 'order' ] = 'ASC';
	return $query_args;
}

add_filter( 'job_manager_get_listings_args', 'xyz_custom_orderby', 99 );

function xyz_custom_orderby_query_args( $query_args ) {
	$query_args[ 'cache-bust' ] = time();
	$query_args[ 'meta_key' ] = '_job_salary'
	return $query_args;
}

add_filter( 'get_job_listings_query_args', 'xyz_custom_orderby_query_args', 99 );