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.
If Rashford is given enough playing time, I'm sure he can achieve great things in the league. As long as he gets the opportunity to play for Barcelona in La Liga, his wing formations are dangerous. He's quick, agile, and a good builder. He can also be dangerous with his crosses. I think Rashford is a key signing, and Flick hasn't exactly trusted him. He believes in his quality.