Question:
Would i have a case?

so i worked as a marketing manager for a water purification company this summer. Worked my @ss off for them, worked there twice a day 7 days a week. I'm going back to school, so i put my resignation in, then 4 days before my last day, they fire me. They received an anonymous letter stating that i lie on my timesheet and never do any work and leave hours earlier than i say i do, which is completely untrue. they fired me on the spot, didn't even look into it. I spoke with the store manager where i worked at and he said that he didn't write it and he guaranteed nobody in his store wrote it. I did find out a few days ago my ex best friend wrote it just to mess with me even though i didn't do anything to her. i told the company i worked for that i found out who wrote it and they said no way, and then they accused me of forging everyone's signatures on my timesheet, the people who say that i was there. the company claimed that those people never were working when i forged their signatures

Answer: First, contact a lawyer... even just to give you a little advice based upon your state's employment (and/or labor) laws. If all they have left to do is pay you $500, if you send one nasty letter on law firm letterhead, they'll probably caugh it up just to be rid of the matter and not spend any money on litigation.

Second, think about backing up your factual claims with written statements from your co-workers attesting that their signatures on the timesheets are authentic -- affidavits maybe.

Third, something seems fishy that they'd fire you right before your last days based upon an unsupported, anonymous letter. This appears to be a preemptive firing for a pretextual reason. It may be that they just didn't want to extend benefits to you under law, or didn't want their retention rates to be low. I dunno. That's why you need to talk to an employment lawyer in your jurisdiction.