If anyone purchased Sunderland, the balance, if in place, would have had to be dealt with in the purchase price, so Donald would actually require a larger sum to cover that repayment on top of his asking price. In other words, any new owner would have to commit to an immediate cash injection into Sunderland on top of what they paid for the shares, in a way that they'd be entirely unable to recover. However, I'm far from convinced that it's actually legally enforceable because of the nature of the transaction; equally, I'm far from convinced that it isn't. It's got expensive law suit written all over it.
Let's try an put this into numbers. Looking at the amount that Donald has put in pre-write off and pre-FPP loan, comes to about 18m (covering the shares and £5m of the parachute put back in). So that's his baseline for the shares. Add on to that the £20m he needs to repay the debt, and you get £38m. For that, the buyer actually gets shares valued at £18m, and a club not owed money with, effectively, his cash in there in a way that can't be recouped from a future sale. That, to me, is deeply unattractive.
With the debt removed (and again, ignoring post write-off movements), Donald could break even on an £18m sale, and the new owners could put capital in as and when if a way they could get back in future. Far more attractive to the buyer. Subsequent movements of £2.5m more coming in as loan, and the FPP loan coming in as share capital have now moved Donald's break even number up to around £30m, but the principle still holds.
Now, it's indisputable that the first scenario is better from the fan's perspective. The club gets all the cash. But the owner is left having paid around £40m for a club worth half the amount. It's not so good for the club, but it could improve the chances of the club being moved on.