n810 is correct.
Per the rule book "a wizard who has a dispel scroll can read it instead of attempting to dispel the spell by using dispel dice." So since using the scroll takes the place of rolling to dispel, if the wizard isn't allowed to roll, he can't use the scroll either.
On page 35, if the wizard fails a dispel roll, "the Wizard is so preoccupied by his collapsing dispel that he is unable to attempt another later in the phase". So putting these rules together, if a wizard fails a dispel roll, he cannot use a dispel scroll later that phase.