While it is certainly true that you did nothing wrong and the error was not yours, it is still the case that you did not pay full rent nine months ago. The landlord is within the statute of limitations to collect back rent.
However, you are not without an argument that the landlord has waived the right to collect the back rent if you can show the landlord knew of the part payment. Florida Statute 83.56(5) says, in part, that: "If the landlord accepts rent with actual knowledge of a noncompliance by the tenant or accepts performance by the tenant of any other provision of the rental agreement that is at variance with its provisions, the landlord . . . waives his or her right to terminate the rental agreement or to bring a civil action for that noncompliance." [partial quote only].
If you want to fight, you could argue waiver or a similar claim of estoppel or laches on the part of the landlord. However, your success in this argument is by no means assured and you could incur legal fees in excess of the amount at issue. If fact, if you have a written lease with an attorney fees provision for the prevailing party, and you lose, you could end up being charged for the landlord's legal fees too.
If you chose to fight, it is very strongly recommended that you obtain an experienced attorney. Please do not by any means think that my answer is legal advice to you or that I am your lawyer. The statements above are just general statements, not addressed to you.
Any response does not create an attorney - client relationship without both parties fully signing a representation agreement.
You have to pay the $990. It's gonna be a hard sell that you didn't notice an extra $990 in your checking account due to error.
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My colleagues are right, but I think you could make a compelling argument directly to your landlord to allow you some time to pay- since they really should have noticed it ahead of time themselves. Unfortunately, dealing with a corporate landlord of a large complex experience is difficult since they're generally less than sympathetic towards their tenants. Tough situation to be in through no fault of your own.