The ṣ substitution does not take place in the s of su (sunoti) in the Future, Conditional and Desiderative.,
Thus :- अभिसोष्यति, परिसोष्यति, अभ्यसोष्यत्, पर्यसोष्यत् ॥
What example will you give under Desiderative? सुसूषति ॥ This is not a valid example, for by 8.3.61, the reduplicate will not be changed here. Then we shall give the example अभिसुसूषते ॥ This is not also a proper example, for it is governed by the restrictive rule of 8.3.64. Then this is the example of अभिसुसूः, निसुसू derived from the desiderative root अभिसुसूषति by the affix क्विप् ॥ Here the desiderative affix सन् is not changed to ष 8.3.61 and therefore the second स would have been changed to ष as it comes after a reduplicate.
Why do we say before स्य and सन् ॥ Observe सुषाव ॥,