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Interesting idea but I can foresee a couple of potential problems if we're looking at being able to bid for another manager's driver mid-contract?
You could be away on holiday for a couple of days and get back to find your driver gone so haven't raced.
You could nobble a competitor by grabbing his best driver mid-season, or prevent him from winning a drivers championship. There are people who would do that just for a laugh...
There would presumably be some sort of compensation to the manager for a driver breaking his contract? If so I suspect it would soon be abused by those wanting a bit more cash.
All of this tbh. The idea isn't really feasible.