Hi,
You can try with the below BTE. It works.
00001120 - DOCUMENT POSTING: Field substitution header/items
Below is the sample code.
READ TABLE T_BSEGSUB WITH KEY TABIX = 1.
T_BSEGSUB-DMBTR = 100.00.
MODIFY T_BSEGSUB INDEX SY-TABIX.
Regards
Gangadhar