That's because we aren't perenially in the CL and have to pay a premium on fees and wages as a consequence, if we can get the player at all.
The situation is reversed when selling to a CL club. See: Gana.
We want to sell (at the right price) because money will go further this summer than it usually does, and because we can use it to buy precisely what we need rather than accept whatever is available in barter.
On the plus side, the opposing party knows that and typically has to offer more value in barter for an in-demand asset than they would in cash in order to compensate for the inflexibility.
We don't want to take back multiple, lesser players because they come with wage bills attached.
Zat's accounting voodoo depends on how amortization works, and I'll admit to not being 100% on the rules with respect to that. I believe that what he's saying is that sale prices are realized now and transfer fees are amortized over the life of the contract, which means that if you mutually agree to inflated fees you both get more to spend today under FFP at the cost of having less available in future windows.