There are fines and penalties for hiring or continuing to employ persons who are not authorized to work in the USA, for failing to comply with I-9 regulations, etc. Similarly, there are fines and penalties for discriminating against someone who is authorized in the USA. https://www.uscis.gov/i-9-central/penalties So, usually the best option for the employer is not to dilly dally in resolving the status of the employees who have an SSN mismatch.