This function module worked fine for me for a similar requirement.
g_netamt type betrg.
G_AMTWORDS type spell.
data :amtwords(200) typec.
call function'SPELL_AMOUNT'
exporting
amount = g_netamt
currency = 'USD'
language = sy-langu
importing
in_words = g_amtwords.
The output - Amount in words will be in g_amtwords-word and the number after decimal point will be in g_amtwords-decword .
These fields can be used in your form or concatenated as given below to a single variable and used in your form.
CONCATENATE g_amtwords-word 'Dollars' g_amtwords-decword 'Cents'INTO amtwords SEPARATEDBY space.