When their finances are in dire straits, Barcelona must prevent a frantic transfer. It would be much better for Barcelona wait one more season until his contract runs out at Bayern Munich than to have to spend a lot of money on a player who is 33 years old. Lewandowski's golden age as a footballer is over, although his performances last season with Bayern Munich were very good, there is no guarantee that he will be able to maintain his form for another 3 years. The age factor can affect a player's appearance, both in terms of strength and sharpness on the field.
The best solution for Barcelona is to wait until Lewandowski's contract runs out, they can sign him without spending a penny.