This puts Barcelona in a bad situation.
They now have to operate under the assumption that Dembele will play for them next season. They could previously maybe pit Dortmund and Liverpool against each other with prices. Dembele not showing up for training now makes it a situation where we can name our price. Dembele is going to try and force a transfer, Barcelona is likely going to come and get him, and we'll get the price we want.
If not, we say no. Dembele can pout for a month, and then he'll apologize, and play out the season with Dortmund. No footballer's gonna sit out a full season of their career. Previously we might've had to make a choice on taking a questionable price for Dembele, now we have all the leverage. We either get to keep the player another year or they meet an overpayment.