Payment Block Status automatically set 'Y' in FBV2

Hi Team,

             My requirement is this,   when user want to post document by T Code  FBv2 or MIR4  

             if user stands on 25 to month end date. means  system date check then

Payment block automatically set to 'Y' (month end posting).

for this one i used many BADI and FM but not working as of now.

one BADI i used as FAGL_SET_DOC   in debugg mode its updating the that fields value

as its technicsl field name is  ZLSPR  whic i set  to 'Y'

its working fine in debugg mode , but when we again check for that Document no then that filed's value is not populating as required.

So please suggest me how can i set Payment block status to 'Y'.

Thanks

Vivek