This should work...
=IF(MOD(J9,7)=0,0,J9-IF(J9<=7,0,D21))

I believe that returns 0 when J9=0, but the OP wants J9 when J9<7.

The formula is wrong for the reasons Duke pointed out, but as to your
statement... perhaps I am missing something in your statement, but isn't my
formula returning 0 when J9 equals zero the same thing as returning J9 when
J9 equals 0?

Rick