I guess a couple of routes:
1 - suing Spurs for whether he should have been registerable or not as a player
2 - Suing whoever did the medical (they would probably have some degree of indemnification)
3 - claiming against insurance
be interested to see what it is.
It's not really the same, but the Emiliano Sala transfer to Cardiff is with the CAS in 2022 to make a decision as to whether they are liable for the transfer fee or not.
The cynical one, which, if we are doing I will immediately stop supporting Inter, would be suing Eriksen for his inability to fulfill the contract, kind of similar to how Chelsea sued Mutu. I very much DOUBT this is what we're doing, but just saying what theoretical options could be. I dont know how players are registered in Italy (though I guess employees given tax rates andd growth decrees etc), but conceptually if Eriksen was a subcontracted company (which I could see in football, at least conceptually, albeit doesnt happen AFAIK at least in the Premiership which is where you'd expect it to happen most), we could sue the company "Christian Eriksen" for breach of contract.