How To Call A Procedure From A Subform In Main Form

I have a sub form which has the Save Button. In case the user keys in the details in the sub form, but then directly clicks the Payment command button of the main form, I want to call the Save button in the sub form if the form is dirty, and do the save, otherwise, proceed with the payment command button.

Every time I try to call the save procedure from the main form's Payment button, I get this error that "object does not support this property or method"

I am calling the save button's code in the subform as

forms.MainForm.SubForm.Save Producedure

Exact Code is : Forms.newpatients.InvoiceHeader.Command7_Click

How do I call this command7_click, which resides in the sub form, in the main form, and invoke it only if the data in the sub form has changed.

Please help.

Thanks,
Vinai