The original sales tax is 31.2% = 7.2%, and the new sales tax is 31.3% = 10.3%.
The price of the article is Rs. 9000.
\[ \frac{7.2 \times 9000}{100} = 315 \]
\[ \frac{10.3 \times 9000}{100} = 300 \]
\[ 315 - 300 = 15 \]
Thus, the difference in sales tax is Rs. 15.