No--your employer may not withhold your pay for work you have already completed for any reason. Certainly, an employer can require a background check before hiring someone; and could elect to fire (or  not rehire next summer) you if a background check is not successfully completed now. However, you must be paid for all work you actually did, and your employer has no legal basis to withhold your pay. You may sue your employer to obtain your money, if the employer will not voluntarily pay you.