Question, Diamond distance as unsuccessful triangle.

I am trying to achieve diamond distance, 500km in the US. Let say I
also was going to declare a triangle with hopes of also qualifying for
the Hilton cup. I declare a 550km triangle, I make my two turnpoints
but do not make it back to my finish point but land out having flown a
total of 520km. Would this qualify for diamond distance? I did not
complete the triangle but I have documentation of having flown 520km.

I know it seems straightforward that it should qualify, but we are
dealing with FAI rules here. Does anybody no for certain the answer.