Modal ‘can’ needs to be replaced with ‘could’ to make the sentence grammatically correct.‘Could’ is used to express power and possibility in past tense. Thus, the correct formation would be, ‘I wish I could help you’.
An electric charge always flows from a body at higher potential to a body at a lower potential irrespective of the amounts of charges contained in them. In the question , no current flows. So there is no potential difference.