conversion frm qty fields to character fields

Hi,

i am working on script

i need to subtract two values (MSEG-ERFMG - MSEG-LSMNG)

i am passing these variables in the perform

PERFORM GET_MAT_DESCREPANCY IN PROGRAM ZMM_SUB_GRN_REJPRINT

USING &MSEG-ERFMG&

USING &MSEG-LSMNG&

CHANGING &P_DIS&

ENDPERFORM

and in the form i am using

FORM GET_MAT_DESCREPANCY TABLES T_IN STRUCTURE ITCSY T_OUT STRUCTURE ITCSY.

READ TABLE T_IN WITH KEY NAME = 'MSEG-ERFMG'.

MOVE T_IN-VALUE TO P_ERFMG.

but i am getting error as T_IN has charaacter values and the 'MSEG-ERFMG' is of type qty

i want to subract MSEG-ERFMG - MSEG-LSMNG values

can u pls help this out