Rent Overpaid

Hi guys, not sure if there is a better sub for this or not so I’m just gonna leave it here.

I have moved country and my housemates and I have ended the lease in our current apartment. We normally pay €1314 for rent(it went up a few months ago but they never contacted us to change the standing order, which is why it says we were in arrears). We had to give 30 days notice which led to us paying for two weeks of the next month as well.
I changed the standing order amount roughly a week before the date it was due to come out, however it still sent €1,314 to the letting agency for some reason.

They are now saying they are withholding the refund until we have fully moved out and handed the keys back(which will be next week) even though they already have a deposit from each of us.

Is this allowed?
Any knowledge/advice would be greatly appreciated. Thanks