I suppose it's not just transfer fees, but also wages. I think de Jong is on something like 10m Euros per year, so if they sell him they also lose that commitment to the wage budget. I think they let Coutinho leave for Villa on a cheap fee mainly so that they could shift the wage burden. If Barca need more money to fund Lewandowski's wages, they might eventually let de Jong (as one of the top earners) leave for cheaper, if they don't have any other options... but for the moment they are seeing just how much they can get out of United.
Maybe that option will also be a consideration for Barcelona in order to loosen their spending on player salaries so Barcelona can release De Jong this summer. De Jong is the player who has the highest salary at Barcelona this season, he is ahead of Sergio Busquest.
Financial problems are indeed something that is not desired by every club in this world, but because of several options some clubs want to cut player salaries by 50% and this has spread news that Barcelona want to cut their players' salaries.
If their financial situation doesn't improve I think one by one their stars will prefer to go to another club this season or in the next season.