I am not sure whether what you've mentioned is a general statement of fact or what is actually happening in your organization. If it is a general statement, it may not be universally true because monetary consideration alone cannot keep an unwilling employee under the services of an undesirable employer. Many more factors, such as job satisfaction, promotional avenues, and general work culture, are involved.
If it is an incident in your company, it indicates a poor state of affairs regarding your employee compensation policy. Had it been an individual issue, you would have no qualms about justifying your proposed monetary concession to an individual employee for the sake of retention. As an HR manager, you are worried about the reaction of other employees, which suggests that your employees are underpaid. Therefore, the real justification lies in dispassionate introspection and suitable remedial measures in this regard.