There are two things you need to do here.

Firstly, share the address that you sent to the other party. We can confirm whether or not he is telling the truth and whether or not he has sent coins to that address.

Secondly, confirm that the address you sent him is part of your wallet and you weren't victim to clipboard malware. In Electrum, either check the "Addresses" tab to see if the address is there, or use the console to type ismine(youraddresshere).