Hi Jyothika,
It has been noted that EL balance in the case of employees having more than 300 leave -----How come more than 300 leaves in a year ?
For your first requirement - better you can choose user exit and ask to abap team to develop the code. before that you know right how to find the user exist ?
CMOD and SMOD both are main tcodes for user exists.
Thanks
Siva