Nabi (PBUH) said: A man asked his friend to lend him 1000 dinars which he gave him. This man then went to the seaside and did not find a shio returning. He took a wood and made a cavity in it. He placed the 1000 dinars in it and threw it into the sea. The companion who lent him the money, came out and found the wood. He took it to home to use as firewood... When he spread it out, he found the 1000 dinars. (Bukhari)

This story teaches us how to fulfil a promise. This is an attribute that every Muslim should have.