adding _ve sign to a field

hi,

i need to add a -ve sign to a field which is of type quen,but the fm i used is taking only chr.this is what i did? but not getting how to do it

loop at i_ekpo into wa_ekpo.

wa_ekpo-menge = wa_ekpo-menge * -1.

data : lw_var(14) type c .

lw_var = wa_ekpo-menge .

CALL FUNCTION 'CLOI_PUT_SIGN_IN_FRONT'

CHANGING

value = lw_var .

wa_ekpo-menge = lw_var.

modify i_ekpo from wa_ekpo.