Is it permissible to say "May Allah have mercy on your dust" to a deceased person?

It is prescribed, upon mentioning a deceased Muslim, to supplicate for him with mercy and forgiveness. The saying to the deceased, "May Allah have mercy on your dust (Allah yarham turabak)," is permissible if it refers to the body of the deceased that has disintegrated into dust. However, it is not correct if it refers to the soil in which the deceased was buried. It is preferable to abandon this saying due to its ambiguity, and instead to supplicate with what is established and clear.