I helped my roommate writing a letter. I am not sure if it's okay. Could someone please be so kind as to have a look an it and correct it? Thank you very much!
And this is the letter:
Four weeks ago we sent you the settlement of the horse "Fury". Unfortunately we falsely included 9% tax. We therefore have to ask you to be so kind as to transfer the sum of €300 back to our account. We want to offer our sincere apologies for that mistake.