Free! dj - Anata no Tonari de, Anata to Tomo ni.

Pairing: Makoto x Haru

From Seabreeze Romance Scanlations:

Haru and Makoto are dating, and all seems right with the world. However, when Makoto forgets a silly childhood promise that Haru can't help but hold close to his heart, it puts a strain on their relationship that both are desperate to put behind them. Will Makoto ever remember that promise? Can Haru forgive him if he doesn't?