purchase user exit

hello,

I am trying to use EXIT_SAPMM06E_013, EXIT_SAPMM06E_016 and EXIT_SAPMM06E_017, for changing the bstae and kzabs fields of one purchase line, i modify tis fields in the table tekpo, but when i return to see the purchase it hasn´t remove the fields, can anybody help me please?

Thanks in advance

My code:

loop at tekpo.

if tekpo-mtart = 'ZSRV'.

clear tekpo-bstae.

clear tekpo-kzabs.

modify tekpo index sy-tabix.

endif.

endloop.