If you think about it, it's quite impossible for Barcelona to buy him if he doesn't show good performance. His sales are still quite high for Barcelona and he also has a high salary, while Barcelona doesn't have stable finances. So I think Barcelona management is only borrowing him for this season as a substitute option and Barcelona has no real intention of buying him.
Rashford was not their main target, it was Nico Williams and I wonder why Williams didn't show much interest and later signed a 10 year contract with Bilbao. From my observations I think Nico Williams is not confident in himself. He feels he might not perform the way they are expecting him to if he moves to Barcelona. That might be the exact reason why Barcelona never pushed more to sign him, despite being the first choice for the left wing. As for Rashford, I have a strong feeling he will do well at Barca.
Thinking that Nico Williams performance was the reason why he didn't move to Barcelona means that you've underestimated him. Barcelona was unable to meet up with the fees expected from Bilbao due to their financial crisis. He signed a ten years deal with Bilbao because his heart is still with them. Don't forget that Nico Williams do play along side with some of Barcelona players in the National team.