there are no pure barters allowed, so the valuation would be in the contract, and should be paid to boca anyway.
the only way around is to sell two players to inda in exchange for barella. let's suppose barella's value is 50m, and our trojan horse beppe would pay 50m for bentancur too. now, we could sell them khedira for 45m and bentancur for 5m, boca gets 2,5m