I was afraid that you might involved into a scam - when the person provides you with the check which you deposits into your account and part of that amount return to the sender via Western union or any other way. A week later your bank informs you that the check was a fake and you are short on the amount returned to the sender.
However - if you know the sender - that most likely not your situation.
You are correct - you will be responsible to report as a taxable income only the amount you earned. So from tax prospective - I do not see any issues.
However - all these transactions do not look clear - and because you are involved - you might want to know more details to avoid any criminal activities.
You might be not privy to his reasons as long as you are not involved, but since you are getting involved - you might want to have more information.
As you have a bank account abroad - please also be aware of reporting requirements.
You may be required to report on the Schedule B part III that you have accounts in the foreign country if the combined value of the accounts was $10,000 or less during the whole year. See details in the instructions to that form http://www.irs.gov/pub/irs-pdf/i1040.pdf on the page B-2
There is no tax associated with this reporting - this is for informational purposes only.
The form TD F 90-22.1- http://www.irs.gov/pub/irs-pdf/f90221.pdf that is referenced on the schedule B should be filled if you have financial interest in or signature authority, or other authority over any financial accounts, including bank, securities, or other types of financial accounts in a foreign country, if the aggregate value of these financial accounts exceeds $10,000 at any time during the calendar year. This reporting doesn't constitute tax liability.
The Form 90-22.1 is due June 30th of each year to report foreign bank accounts owned in the previous year.
Let me know if you need any help.