Enter amount in GRN

Hi,

I have one requirement, it looks like crazy but still customer wants it due to some administrative reasons.

Example : Create the PO for 2 Qty - 2000 USD

GRN need to do based on the value Ex: received 500 USD, they want to enter the Vaule at the time of GRN, once they enter the value as USD 500 system needs to calacute as 0.5 Qty based on the GRN value.

By creating the New movement type i think we can open the amount field in MIGO, but how to calaculate the Qty based on the Amount entered??? is there any Exit or BADI to achieve this.

Thanks ,

dhamodar