Obligation to buy unless outbid by a certain date would be even better. And an additional right of refusal premium to cover that situation.
That way you can set a price that seems reasonable now, but when he shows his inner Koulibaly we can get more.
Also options for Man Utd to cancel the obligation, repurchase clauses, sell on clauses and 10 options to purchase any u21 prospects for no more than £10m.
And an entire ban on transfers from Fulham to any big 6 rival without us having the option to take said player for a 50% discount.
That's how you structure deals......