state - alabama-- salaried employee resigns

i worked for a company that paid salaried employees on the 15th of each month regardless of when you began employment. after six weeks i resigned as it was obvious the job was not what i expected. the resignation was effective on the 16th of the month, one day after i was paid for the month. about two weeks after my termination, i was sent a certified letter stating that i owed the company for the days of the month i did not work. if i was a salaried employee, why do i have to pay money back to the company?