those salary slips and bank statement will be attach by return file either by you or any tax lawyer you hire and by foreign income you are not have to pay tax in pakistan.
Same is the case happend with my father. He worked in kuwait for his whole life and after retirement he came back to pakistan in 2012 and in 2013 he got one of his plot commercial registered with some fees deposit and proper channel. Later in 2014 he received a letter from FBR that you are not a filler and what is the source of income and how you made this property. They give us some date to provide the evidence or they will seize the account or blah blah...
Than we contacted one tax lawyer he said if you sent all your transaction from abroad through bank account than it have in statements. That lawyer fill the return file with bank statement with my father first Passport on which he traveled with departure and arrival stamp page. Now all set. He just need to return the tax file annually and no tax alleged on it as his money is foreign based.
there is minimal lawyer fee that we pay to lawyer annually at time of file return and he is now NTN register.