Overapplied Credits test case

Hi ,

I need some help from you all in testing the "Overapplied Credits " case  in our testing. This is what I have done so far.

1. Enabled the "Allow Overapplication" check box on projects credit memo transaction type in AR.

2.Created an event for 800 , generated draft invoice and interface it to AR.

3. Tieback to Projects.

4. Now I trying to create a credit memo from draft invoice for an amount 1000 ( actual inv amount is 800). System is not allowing to enter amount greater than balance amount on invoice.

I tried to create a -ve event for an amount to -1000 and transfered it to AR, its going to AR as -ve invocie but not credit memo. So I cannot apply on the actual invocie.