How can we cancel Prepayments created to Employees which are now Ex-Employees?

We have many prepayments which were created to employees. The prepayments are available and they were not applied to any invoice, because the Employees were fired.

The employees should have created the expense report before they were fired, but they didn't.

Now, Oracle doesn't allow us to create expense reports or invoices to ex-employees. So we can't apply the prepayment, either.

What can we do? We don't want to turn ex-employees into employees.

Regards

Einer