Only legal contracts are enforceable in a court of law. If the consideration given either way was patently illegal e.g. giving a bribe, accepting a bribe, buying political influence, abuse of office, etc, then Hillary’s “clients” have no legal recourse and simply have to absorb their losses.
So the only legal grounds anyone would have is based on something along these lines:
"I contributed to the Clinton Foundation because they said they were [helping people in Haiti, helping people in Africa, etc.], but they just used my money to pay their own salaries and pay for Chelsea's wedding."