Have you requested "proof of income tax payment" (for immigration purposes) from FBR? Or if your salary is under the income tax threshold, a "certificate of exemption from income tax"?
If you pay income tax, you must have (or be able to obtain) some documentation proving that? Then get it stamped and certified by the relevant authority?
(I know Pakistan is a tough country to get these documents, have you asked Immigration QC what documents they will accept if you are unable to obtain the above? They will, I assure you, already be aware of the difficulties.)