The original sales tax is \( 3 \frac{1}{2} \% = \frac{7}{2} \% \), and the new sales tax is \( 3 \frac{1}{3} \% = \frac{10}{3} \% \).
Step 1: Calculate the original sales tax on the article priced at Rs. 9000:
\[
\frac{7}{2} \times \frac{9000}{100} = \frac{7 \times 9000}{200} = \frac{63000}{200} = 315
\]
Step 2: Calculate the new sales tax:
\[
\frac{10}{3} \times \frac{9000}{100} = \frac{10 \times 9000}{300} = \frac{90000}{300} = 300
\]
Step 3: Find the difference in sales tax:
\[
315 - 300 = 15
\]
Thus, the difference in sales tax is Rs. 15.