checking for business area while posting

i have to check for business areas balancing during posting document

eg., when we get interest income from a bank account under business area X it should be credited to an account with same business area X.

Dr Bank - X

Cr Interest income - Y

this should not occur

so user wants to get a error message when the business areas does not match.

Can any one tell how this can be done?(does fi validations help in this?)