For me, the Telegraph reports about his deferred salary from earlier today are the most believable reason as to why this is dragging. If you leave aside all the other speculation and bullshit and just consider this alongside the actual soundbites we've had from Laporta and FDJ (because to my mind, the only things you can put any real stock in one way or the other are actual 'on the record' quotes), it is starting to look like brinkmanship between those two camps.
Surely any deferral of wages will have been contractually agreed and signed by both parties... nobody is making gentleman's agreements with those sorts of figures involved. That means there is an agreed repayment plan that is likely complicated by FDJ leaving the club. I'm no expert on Spanish employment law but I cannot see how Barca can refuse to pay him the deferred money just because he has left unless the contract explicitly says that he waves any future payments if that happens. However, I also cannot see any semi-competent lawyer or agent advising the acceptance of a contract that could, under the right set of circumstances, ultimately leave his client significantly out of pocket in future. I guess it could happen, but I'd be surprised if he has ended up in that situation. Barca agreed to pay him money for 'services rendered'... those 'services' have already happened so Barca owe him at least some of that money somewhere down the line. Perhaps the repayments are covered by some sort of clause that says if FDJ seeks to initiate a move away from the club, Barca are not liable for the back-pay, much like a loyalty bonus, and that is why we are seeing the back-and-forth between Laporta (saying Barca aren't pushing him out) and FDJ (positioning himself so as not to breach such a clause). There could be no clause of that kind and Barca are simply chancing their arm by saying "of course we'll agree to sell you but only if you agree to wave (or maybe reduce) the deferred payments" and FDJ is quite simply holding his ground... he has all the cards in that scenario and little to lose so why not. That would be what I would do, especially if leaving could in any way be legally construed as tacit agreement to forego some or all of the money owed.
I am not up to speed on the ins and outs of Barca's finances, what these magical levers have done for them both now and over the next 5 years, or what is happening in terms of pay cuts and deferrals throughout their squad. I also don't know much about La Liga's FFP/wage cap rules. There is however too much smoke for there to be no fire at all with this one... United are seemingly chasing the player but keeping extremely quiet. Barca are seemingly talking to someone about a FDJ transfer, even if they do also believe that they may keep him. I am starting to think that if he stays, it will almost be out of spite because Barca tried to mug him off to the tune of millions of euros and he refused to cave to their demands, or his agent/lawyers couldn't find a way for him to leave without seriously affecting the money he is owed. If even a fraction of the reports about Barca needing his wages off the books are true though, I think he will leave at some point soon and it is purely posturing on both sides to put themselves in the best financial position. Unfortunately, if any of that happens to be in any way accurate, United are caught in the middle until they sort it out.