Reading between the lines of the stories doing the rounds at the moment it seems United have the money but due to fans not being allowed in the stadium finances are tight, especially considering we don't know when fans will be allowed to return. There's also a concern that we could see a second wave and another lockdown around Christmas, which would mean clubs have to pay back TV rights they've been paid again.
I can understand the concerns from the club when there's this much uncertainty around the finances this year. I'm still confident this will happen though, United will pay it eventually, I just think it will boil down to Dortmund taking a deferred payment if there's a lockdown again or if no fans are allowed this season.