Talk about oversimplifying a hugely complex financial transaction!
It’s a little more nuanced than that mate, particularly with an organisation as clearly in the financial shit as Barca.
Think more of the deal Leeds struck for Raphinha - yes there will need to be a chunk up front, as you say, but there will also need to be clear evidence of their capacity to pay across the structure of the deal - so when each instalment is due, they can demonstrate the money will be there. And if not - clear penalties written into the contract for non-payment at each stage.
This isn’t as simple as saying - we’ve got £25m, let’s do a deal for £80m and worry about the rest later.
You’re right that it all depends on de Jong though - providing they can flog him to Chelsea asap, I’d imagine a deal will get done.