From a practical standpoint it shouldn’t be a problem as Jim stated but depending on how you apply the NEC rules it could be a violation.
Let’s say that the unit has an MCA of 30 amps and a MAxOCPD of 40 amps. The conductors are #10. So far so good but the conductors from the service panel to the fused disconnect are feeder conductors by the Article 100 definition so the 50 amp OCPD cannot be used to protect the #10 conductors. You could apply the tap rules and possibly still use the #10’s.
Now if we call the fuses supplementary equipment protection then IMO you can use a 40 amp OCPD at the service with the fuses at the disconnect but to use a 50 amp OCPD the tap rules would need to be followed.
However, all of this goes out the window if you use a 50 amp conductor.