Return a sale generated by sale opportunity

Hey Tryton community!

I was doing some test in tryton-demo, and I created a sale opportunity. After the sale created by itself, I processed it. The main problem is the sale can not be returned. It shows the following message:

You cannot modify the opportunity origin of the sale “XXX”.

Why I wouldn’t return this sale? Perhaps I’m wrong, but it is another option to return it?

Thanks in advance, you all.

Please fill a bug report.