[geekishmatt]

Sometimes a subtraction of a unit absolutly legal and you are still in the same model or context. (Temperature loss)

Sometimes a subtraction of a unit is hidden-black-magic and you are switching between two models or contexts but still being in a meta-model or meta-context. (Debits and credits)

It just depends on your model or context how you interprete your value/unit relation, but this is a typical problem in applied physic, engineering or computer science :)