Visit visa for mother

[mash1234]

Hello,

My mother's entry clearance visa got rejected for Family visitor Visa last week. My visa status is ILR and I was sponsoring my mother.The reason they mentioned in the visa rejection letter that they are not sure from where £7000 came to my mother's bank statement.However, my mother clearly mentioned in the cover letter about the source of the money. The money came from a sale of a flat in 2001 which she owned. After that, she kept the money in one financial organisation for the last 5 years. She has provided the flat sale documents and the deed of agreement with that financial organisation where the money was kept for 5 years. It seems like the case worker completely overlooked the documents and cover letter.

As there is no appeal system, should she make a new application stating previous mistakes they did?
Or she can complain by sending any email?

Please advise.

Thanks in advance.