Found it, this from my original post just before he signed, it came from the As editor, so a decent source...
Article 18bis - "Third-party influence on clubs" of FIFA's "Regulations on the Status and Transfer of Players"
No club shall enter into a contract which enables the counter club/counter clubs, and vice versa, or any third party to acquire the ability to influence inemployment and transfer-related matters its independence, its policies or the performance of its teams.
The fear is that clubs use such clauses as a way of getting round FFP and other financial restrictions and regulations. For this reason, when Madrid sold Achraf to Inter earlier in the summer, the clause was a "right to first refusal", not a unilaterally enforceable buy-back, where the new club and player have no say in the matter.
That last paragraph is mine, summarising the article in Spanish