Negative moving avarage price

Hi,

since I work in company where we consume garbage materials, which we purchase at zero value and for which we are paid, we should manage negative values on stocks. That means at the end of the period you could have positive stocks with negative values.

Do you know if it is possible to manage it in SAP. Apparently it is not possible because if I try to make a price change of a material by using MR21 transaction, the system displays an error which doesn't allow to input negative MAP.

Please advice

Regards

Dario Corno