[[coso]]

Entonces, en el evento OnDblClick o bien OnClick, deberas recoger las variables y luego enviarlas.

Código Delphi [-]
procedure TForm1.DBGridOnDblClick(sender : TObject);
var
    cod : integer;
    dsc : string;
    prc : double;
begin
    cod := (sender as TDBGrid).FieldByName('codigo').Asinteger;
    prc := (sender as TDBGrid).FieldByName('precio').Asfloat; 

    if (sender as TDBGrid) = DBGrid1 then // DBGrid1 = la de servicios
    dsc := DBGrid1.DataSource.Dataset.FieldByName('servicio').Asstring
    else 
    if (sender as TDBGrid) = DBGrid2 then // DBGrid2 = la de productos
    dsc := DBGrid2.DataSource.DataSet.FieldByName('descripcion').Asstring;

    Insertar_en_factura(cod,dsc,prc);
end;