Schuster has revealed ex-Real president Ramon Calderon was focused totally last summer on prising Cristiano Ronaldo away from United and refused to consider alternative signings.
Ferguson dug in his heels and fought off Real's very public pursuit - which included support from FIFA president Sepp Blatter - of the Portuguese star.
Schuster revealed: "I said to the president, rather than try to sign Cristiano Ronaldo, we needed an additional three players. But the club were relying on him and if it happened, it would have been difficult to pursue other reinforcements.
"But the most disappointing thing in the end, is that nobody ended up joining. There was money for Cristiano Ronaldo, but not for (David) Villa, (Cesc) Fabregas or (Daniel) Alves for example."