Let say you owe the bank 500k in mortgage. What is more affordable between paying off in one go interest free, or you pay them over the years interest or not?
Very basic accounting, when you want your revenue as high as possible regardless how much net positive you end up with. Because higher revenue, cashflow would help future loan. Lowering the fee for them is less attractive than having FdJ loan them the money they owe him.
FDJ isn't a bank, he's an employee who has a contract, there would be no need for him to offer a multi billion valued company a 17m loan.
It wouldn't make any sense for him either unless he attaches an applicable interest rate as money today is worth more than money tomorrow and considering inflation is crazy right now, that coupled with Barcas seemingly precarious position and the lack of ability to take on more loans, 10-15% interest wouldn't be crazy to suggest.
So that would be between £51.7m - £72.7m over 30 years (mortgage comparison you made) from a £17m loan - even then FDJ would be stupid to offer a business thats selling off their future revenue for short term gain a loan.
He's on a legal higher ground, he extended enough goodwill to his employer by differing his wages in the first place, and if the law were to get involved he'd have an easy win with the evidence of Barca splashing the cash on new signing.
If you owned a business and had a clear paper trail of buying expensive homes, cars and holidays yet consistently failed to pay your debt you'd be rightfully thrown in jail (well, a normal joe would declare bankruptcy, but Barca obviously wouldn't do that).