I'm not aware of there being an accounting standard on football transfers, so I'd expect the accounting to be based on normal principles.
On that basis I'd expect that the entirety of the "fixed fee" (i.e. not dependant on anything happening in future) would be recognised when the contract was signed. This would include any element with extended payment terms, with the "other side of the entry" being shown as a debtor on the balance sheet until paid.
As noted above, if the buying club then defaulted on the extended term payments, this would be written off as a bad debt, so reducing profits (or in most football clubs' cases, increasing losses) in the year of default.
I'd expect any "add on" payments (i.e. dependant on something happening in the future) to be treated as contingent assets and not be recognised in the accounts until the relevant event(s) happened, because of their inherent uncertainty.