Problem with Man Utd is that they're going all in for De Jong for now, whether they will be able to get him or not will greatly affect to their transfer budget. De Jong's estimated price would be 70-80mil€, that would leave United 60-70mil€ for remaining transfers, if their rumoured transfer kitty of 130-150mil€ is true.
After possibly capturing De Jong their priority would be to reinforce other areas (striker, DC) ahead of right back. So selling Dumfries to United for 40million'ish is not gonna happen in an instant, we'd be needing to seek out other suitors to get that quick deal if we need it.