That seems to be what they were planning behind selling Frenkie de Jong. Yes, they will use that profit to buy another player, namely Lewandowski, whom they have been eyeing for all this time. But Barcelona must prepare a replacement for the player who played regularly last season. But if they are a little late to sign Lewandowski, they could lose him too, because now rumors about a team being interested in Lewandowski are getting louder.
I want to know which other team wants Lewandowski, indeed all the teams currently want Lewandowski but in reality Lewandowski only wants to play for Barcelona. Lewandowski himself wanted a salary increase at Bayern Munich but unfortunately Bayern Munich did not agree to that, which is why Lewandowski was so adamant about leaving and the team he wanted was only Barcelona and no other team. Although there have been rumors of interest from other teams, those rumors have always included a refusal from Lewandowski.
There are a few who are interested and are actually quite capable of bringing this player in but indeed, as you said, Lewandowski now only wants to be at Barcelona and obviously it will be difficult for other clubs.
But the problem is what is Lewandowski looking for in Barcelona because if he decides to leave Bayern because of salary obviously Barcelona is not a good option to leave. Their current condition does not allow Barcelona to give lot of money to Lewandowski.