That's not how it works.
Barcelona won't bid more than 40m. That's stupidity.
They just need to bid 40m. And wait for Rashford to turn down Villa contract offer, then back to United accepting Barcelona offer and Rashford agree
They cant and dont need to outbid Villa 40M because they have first option, and Rashford would have picked Barca if given the choice. If they're being an arse they could have bid 20M and expect Rashford to push for the move since the relationship is already untenable.
How can you stipulate a contract when Villa only has an *option" to buy and it hasnt been exercised, technically Rashford is 100% United player. He can't and hasn't signed any agreement to move next year, hence it's an option to buy and not an obligation to buy
So Rashford still has to agree moving permanent to Villa, including a new wage negotiation. If that doesnt work Villa will send him back to United.
Barcelona knowing United already wanted to sell for 40M to Villa, wont stupidly bid 70M
So unless United has a personal grudge with Barcelona and refuse to sell Rashford for 40M, they will get Rashford. And Rashford will push for the move if it's Barcelona.