Remark
Are those Euros that he used to to sell his tables for? I can't quite make it out.
Anyway I will take the price to be 12460. If I have anything wrong, either change it or notify me with a comment.
Gain
20% of 12460 = 20/100 * 12460 = 2492
Total selling prince = 12460 + 2492 = 14952
Loss
20% of 12460 = 2493
Total selling prince = 12460 - 2492 = 9968
Net Gain or Loss
His net gain is 0. What he made on the first sale, he lost on the second one
He should have gotten 12460*2 = 24920 without the 20% gain or loss.
What he got was 14952 + 9968 = 24920 which is a net gain or loss of 0.