I'm no lawyer, but I'd be surprised if this is literally true.
One the one hand, if Trump says that Hillary has sold influence, she would be unwise to sue for slander because whatever came out would make her look bad. Suing for slander is a bad idea in this case.
On the other hand, if Trump says that Hillary had served time in prison for molesting children, I think she could sue for slander and win. It's a lie. I don't believe that it is flat out not allowed for Hillary to sue for slander just because she has put herself out for approval.
In the case of calling her a child molester who served time in prison that would probably fall under “malice aforethought”. If Hillary could prove that then she could win. And I’m sure that would be Trump’s reason.