Is the sentence correct?

Had it not been for the fact that we had our old flat sold quite quickly, we would have not been able to afford to buy the new dwelling.

...would not have been...

>>>for the fact that we had our old flat sold quite quickly,
...for the fact that we had sold our old flat quite quickly,

someone sold it for them?

I think so.IT'S CAUSATIVE,SO IT MUST BE CORRECT.
But you didn't answer if the whole sentence is correct.

Z poprawka mg wydaje sie poprawne.