Exempt Salary Manager that worked 21 paid days, yet employer only paid for 18 and refuses to pay out

I am a salary exempt manager for a food service company. The month of August was a 21 day pay month (meaning I would need to work a minimum of 21 days to receive my salary. I am new to the company and had a previous vacation planned for the end of August. My employer stated that the vacation would be unpaid (due to time served) which I fully understood. During the month of August, I was asked to work 15 days in a row with no day off (to open a new account) and forfeit 3 days from my unpaid vacation, which I did without hesitation. At the mid month point, I turned in my time sheet to my supervisor for approval. On the time sheet i listed the 21 days that I was working (including the 15 days in a row and the 3 vacation days forfeited) along with 4 other days listed as vacation. When i totaled the days worked on the form it came to 21 days (which did not include the unpaid vacation days). No issue was brought to my attention. Upon my return from vacation, I received my paycheck, which was only for 18 days. When I asked the payroll manager and COO of the company, I was told that the unpaid vacation days were deducted from the 21 days worked. I explained that the unpaid vacation days were not included in the 21 actual worked days so i should have been paid for the 21 days that were actual worked, regardless of the 4 vacation days that were to be unpaid (just for the record my vacation was to be 7 days, 3 of which I physically worked). the employer is refusing to pay stating that I am not authorized to work "extra" days for comp time. I explained that these days were not "extra" and were required for the job.