BAPI: unit translation and material format not applied in WD Application

Hi all!!!

I am using the BAPI SD_SALESDOCUMENT_CREATE. In R3 I have a unit of meassure called PZA (it represents "pieza" in spanish), but in database is stored as ST. I am creating items through the menctioned BAPI but it is returning me the meassure unit as it is stored (ST), so the desired value to be returned has to be PZA.

Please help me on how can we achieve this.

thanks!!!!!!!!