If, after submitting your work, a client asks you to go back and complete something that was NOT in the original instructions please do one of the following:
If you don't mind going back and completing the extra work, feel free. We support you in going the extra mile. Make sure to click the "submit your work" button AGAIN to submit your extra work. Performing this additional work for the client could prove how reliable you are and they may add you to their private workgroup.
If it doesn't work for you to do the extra work (you are in no way required to), you have the right to professionally and politely decline the additional request. Explain to the client that you have fulfilled the contract as was requested in the instructions. Ask them to mark the gig complete, and be sure to click the "submit your work" button before your deadline expires so that the gig will be back in the hands of the client and they can pay you.
Note: There are a couple of things you can do proactively to help reduce the chances of clients requesting additional work:
If the instructions are unclear, message the client as soon as you are accepted to the gig, asking for clarification.
If you get to the gig location and things are not as you expected, so you cannot complete the work as requested, it is always a wise idea to take a photo showing that you were at the location, even if the client did not specifically request it. If you're looking for a display and it cannot be found, then it would be great to provide a photo showing where one might expect the display to have been, proving that it's not there. Read more: https://gigwalk.zendesk.com/entries/27087630