Increase / decrease in material stock while downgrading

Dear Experts,

Could you please help me in finding a best solution for the below two scenarios?

1. Material A has been received initially with 100 MT, which has suffered from a damage of 2 MT. Due to the damage the 2 MT will be downgraded from material A to material B. But after completing the downgrade process the quantity of material B should be 1.5 MT (decrease in quantity).
2. Material A has been received initially with 100 MT, which has suffered from a damage of 2 MT. Due to the damage the 2 MT will be downgraded from material A to material B. But after completing the downgrade process the quantity of material B should be 2.5 MT (increase in quantity).

I'm aware that we can use material to material transfer (mvt type 309) process to downgrade a material, but here the quantity will not be varying, meaning, if i transfer 2 MT from material A to material B, material B will be receiving the same quantity i.e. 2 MT. But how can we capture increase / decrease of quantity while downgrading?

Kindly help me to implement a best solution.

Thanks & Regards,

Pavithra