jmc

Denote the original portions for Al, Betty, and Clare as $a$, $b$, and $c$, respectively. Then $$a+b+c=1000\text{and}a-100+2(b+c)=1500.$$ Substituting $b+c=1000-a$ in the second equation, we have $$a-100+2(1000-a)=1500.$$ This yields $a=\boxed{400}$, which is Al's original portion.

Note that although we know that $b+c=600$, we have no way of determining either $b$ or $c$.