Ask the person for the payment, if they refuse then contact a Gaian Mod and explain the problem...

In the future, however, do not give them the item of purchase untill you have the payment first.

If they insist on seeing the work before paying you, then go to the image you have saved and copy it into a blank window of the same program.

From there open up your text box and choose a large sized font before writing "SAMPLE" over it. Then save it as a COPY version of what they will get.

Send that to them so they may inspect the work. After they pay, send them the original version; the copy that does not have the word "SAMPLE" on it.