Retainage against Credit Note

Hello Gurus

I have below scenario in AP module

1) Invoice is booked for $100 with 10% Retain-age

2) Retain-Age is released by AP Team

3) Month later , We have received Credit note from Vendor for 50% Amount

4) When CN is booked for $50 , retainage is booked on CN for $5

Question is how do we release this retain-age for negative amount as application is not accepting negative amount when i select Release Retain-Age as Invoice type

Thanks in Advance

Thanks

Niileysh