The problem is Chelsea don't actually want to pay 120M€, let alone more than that to compensate for the payment schedule. I'm pretty sure even the structure mentioned by
@TheMagicFoolBus involving player exchanges is not true. At least according to Fabrizio Romano our offer never actually went past 85M€ in the previous round of negotiations.
If what Fabrizio was saying is true, the talks breaking down had nothing to do with the payment terms but the actual size of the offer instead. Apparently we're still not willing to pay the full 120M€ so if Benfica still insists on that the move is simply not going to happen.