PSR is looking at your accounts on an annual basis though, if we kept him for the remainder of his contract we're spreading the expense across 3 years.
Let's say his book value is £27m in the summer. And he has 3 years left on his contract. We can either keep him and incurr an expense of £9m for the next year.
Or we sell him for say £10m, we'd incurr a £17m expense for next year because that's the loss we've made on his book value.
Sure you save on Onana's wages but the keeper you bring in to replace him will have wages. The point being, if you sell your goalkeeper and bring in a new one, you then have the amortisation of the new keepers transfer fee plus the £17m loss you just made on Onana, all incurred in one year's accounts.
It's not as simple as you can never sell at a loss, but you can't make a habit of making big losses on players book value.
If we can get a decent keeper on a free or a small fee that might solve a lot of our problems. And maybe we can convince some sucker to overpay for Onana so we only make a small loss instead of a big one.