I think in reality they'd be able to pay it. Sartori is seemingly getting more involved now and he's got plenty, I imagine Donald also has plenty given that he had to show £50m to the EFL when buying us.
Assuming it's a secured loan to Madrox I think it much more likely that the plan is to pay it back in shares after a set amount of time. Basically, from what was coming out when the takeover seemed to collapse, it appeared that FPP were anxious about the risk. They only really make money if we get to the PL but they're seeing the likes of Leeds struggle (similarly big crowds, but their tickets cost more so they'll have a big gate receipt advantage).
This is a way of minimising the risk. Donald bought the club for £15m. He likely wouldn't want to sell it for that as it would mean all his work was for no profit, but FPP might think it's a fair value given the aforementioned risk. So they loan £9m to be repaid in, say, 3 years. This gives Donald and co a chance to maximise their profit. If they do rubbish and, even with this investment, we are still in league one, then £9m would likely give FPP 75%+ of the club, so they get it canny cheap which helps offset the money they'll have to spend to get to the PL.
If Donald does as well as he thinks he can and we're an established Championship side by then, the club might be worth £50m. So the £9m only gets them under 20% of the club. That's a result for the current owners because they still own £40m odd of value that they bought for £15m. They can either sell the rest to FPP then and make a good profit, or sell some and retain the rest to hopefully make an even bigger profit if they can get us to the PL. For FPP it means the deal costs them more, but it comes with a lot less risk, they've got a Sunderland that likely won't need hundreds of millions to be back in the top flight where its value will rocket, so they avoid the risk.
They could have bought £9m of shares now, but if they'd done so and the club didn't get promotion and was ran badly (they won't be able to be hands on so will have to trust others) their £9m would lose value as the overall value of the club falls. This way they definitely get £9m back, either in cash or possibly in an increased number of shares if our value has fallen, and it lets them see close up if they really want to be involved. If that's what they've done (and I'd think it likely since they aren't a bank so won't normally be in the business of loaning money just for a slight return), it is a very good bit of risk minimisation for them, and I'd think that the current owners have done very well for all concerned getting them to agree to it