We Were Told Our Rent For 1 Month Would Be Reduced. Now They're Trying To Charge Us For The Rest

My wife and I live in a trailer park in Iowa, and we just moved to a different trailer in the park because Old Trailer had many problems. New Trailer would be an extra 300 dollars per month, but we were okay with paying that.

So we were supposed to move in at the beginning of December, but that didn't work out because they had a few things to finalize before they were ready for us to move in. Now, because Old Trailer had problems, before we moved, we asked the trailer park manager, Amy, if there could be anything done about the amount of rent we owed. She looked into it and found out that she couldn't do anything about that for the month of November, however, for December, we would pay the amount for Ops Trailer instead of New Trailer. And we were perfectly fine with this plan, especially because we didn't even get the keys to New Trailer until December 20th.

So for December, we owed about 1,000 dollars, and we paid it. Now, a few days ago, we got the notification that we had rent due to pay by the 6th of February, which is normal. But we noticed it was more than it should be. So we looked into it, and it was saying there was an extra amount of 300 dollars being sent to us for the previous month. We made sure we paid the full amount from last month(1,000), and then we called the trailer park manager, Rick(this company goes through managers like hot cakes). He said he'd look into it. A day later, my wife called him and asked about it, and he said that that extra 300 dollars was for December because we only paid 1,000. My wife tells him we were told by Amy that we would only have to pay that much, and he tells us that the big bosses say she shouldn't have done that. And also, the cherry on top is that if we don't pay this extra 300 dollars by tomorrow, they will give us an eviction notice.

The way I see it, whether Amy should charged us the lesser amount or not, she did, and that's not on us. We have an email saying our due rent for December was what we paid, and we have bank statements proving we paid that amount. So I'm wondering if the trailer park can even claim we have to pay this amount. We're also pretty mad about the fact that they're threatening an eviction notice even though they didn't even tell us they were gonna charge us that extra 300 dollars. We only found out because we called and asked. And now they're saying if we don't pay we're getting evicted. Acting like we've had this amount due for a whole month. Even though we just found about it on Tuesday, and didn't find out what it actually was until Wednesday.

Is this legal of the trailer park to decide we have to pay, even though we were told we wouldn't have to? Is there anything we can do/look into to fight this?