What would you do if your apartment complex demanded this from you?

So, I had recently moved into an apartment with my s/o and dog. We loved the place and was excited to have it. We were notified of all charges in the beginning (security deposit, dog fee, etc.) And it was listed on one of the final documents we signed. Well there was all these pro rates taken into an account and we... show more So, I had recently moved into an apartment with my s/o and dog. We loved the place and was excited to have it. We were notified of all charges in the beginning (security deposit, dog fee, etc.) And it was listed on one of the final documents we signed. Well there was all these pro rates taken into an account and we added up everything and it seemed correct so we payed all the money the day of move in. (The communication was HORRID, we found out that we got accepted 5 days before move in, we had to physically show up to ask, and her response was "oh youll have time" and we work 50 hour weeks.....) anyway he get a notice in the mail sayign we owe $350 due by the following week, no explanintion...ok no? We call and they said they messed up the final costs and excel miscalculated the total for the dog fee and they need the $350. Now I was PISSED as it through off my check book balance and I originally thought i had extra money and spent it on new apartment things. We settled on paying it before July 1st, even though i dont believe I should pay it as it was their mistake. Anyway, I get another notice saying the payment is due by JUNE 21. Now, im just livid, I call and they said they need it by then so they can log it in the books (to save their own asses) and I grew so upset I am now resorting to upper management. Am I wrong? What would you do? (There had been multiple miscommunications on their end I just dont have the energy to type it all out.)
Update: We had never agreed on June 21, she previously said before July 1st is fine, meaning I had up until June 30
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