This thread is full of misinformation. Anyone with little or no knowledge will end up confused going through this thread.
So you earn money, you have to file tax return to make it your legal earnings. If you are only getting foreign remittances, then you don't have to pay tax, But you have to file tax return.. to show that you earned this much amount and liable of 0 tax on it.
If you don't file tax return and wealth statement, you can get away with small amounts and transactions but moment you will spend your money to buy something expensive ( a plot, a car etc ), you will get into radar of FBR and FBR will come up with questions.
I have been working online for last 12 years, getting all payments as foreign remittances, got encashment certificates of every transaction, filled tax returns with ZERO tax every year. Have been selected for audit as well. Had no issues at all as I had everything documented and filled.
regarding general rule mentioned by "Nabs". I have got transactions $10k and above many times without any questions asked. why? because I file tax returns.