delete the current sub form. make an actual form based on the same record set. give this new form a header/footer and create the sum using an unbound text box in the footer of the form. the header is not needed and can be not visible.
then insert this form into your main form as a sub form
finally your Amount text box in the main form should be unbound and its control source will be: =[SubFormName].Form![SumTextBoxName]