Maybe this one? :) Its about schoolguys and the seme tells the uke that a girl is very persistent and wants to be together with him (seme). So he asks the uke to be together with him so that the girl leaves him alone. --> But in truth the girl wants something from the uke and the seme actually likes the uke ;)
http://www.mangago.me/read-manga/ordinary_crush/
Guess this manga would fit your condition:
Masami loves his best friend Shirou and when he caught Ryuuji ogling him, they make a deal - no one is allowed to attack Shirou and they relieve each other to endure. But Masami made the wrong conclusion, Ryuuji is not in love with Shirou but with Masami!
http://www.mangago.me/read-manga/renai_kyoutei_nukegake_nashi/
Can anyone give me a recommendation yaoi manga that the uke being tricked by his past lover or by the seme?