It is obligatory to perform the farewell Tawaf before leaving Makkah. However, if you have left before performing the farewell Tawaf, you are required to expiate by offering a sacrifice in the Haram. You can do that yourself or by sending the money to someone for that purpose or you can entrust an honest company to do that on your behalf.
Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, states that: If you perform Hajj then you should not leave Hajj until you have performed the farewell Tawaf. If you leave before performing the farewell Tawaf, you must offer a sacrifice to be slaughtered in the Haram; and you should not eat any of it; rather, it should be given to the poor, because the farewell Tawaf is obligatory after Hajj. This is based on the general meaning of the hadith of Ibn `Abbas (may Allah have mercy on him): “The people were commanded that the last thing they should do is to circumambulate the House, but an exception was made for menstruating women.” (agreed-upon hadith). Therefore, you have to repent to Almighty Allah for leaving before performing the farewell Tawaf.