So the set $M$ consists of all the $7!$ permutations of $1346789$, therefore the difference of any 2 members should be $0\mod 9$ Since $d>0$, the smallest value is $d=9$, which can be attained by $134687-134678$ Since none of the digits are $0$ or $9$, subtracting 9 will never affect more than the units and tens digits (in fact it will always only affect these 2 places). Moreover, it can be seen that if the last 2 digits of $y$ are $ab$, then the last 2 digits of $x$ must be $ba$, and that $b-a=1$ So $b$ can be $4,7,8,9$. There are $5!$ permutations of the other 5 digits, so the total number of pairs is $4*5!=480$