The Prophet (may Allah's peace and blessings be upon him) bought a camel from him, and he weighed out (the price) and paid him more than his due

There is a story behind this Hadīth, and it is mentioned here in brief. The Prophet (may Allah's peace and blessings be upon him) bought a camel from Jābir (may Allah be pleased with him). "And he weighed out for him," meaning: the Prophet (may Allah's peace and blessings be upon him) weighed the price of the camel.

This is said figuratively, since the one who did the weighing in the Hadīth was Bilāl (may Allah be pleased with him) upon orders from the Prophet (may Allah's peace and blessings be upon him) as stated in the original Hadīth: "So he ordered Bilāl to weigh for me an Uqiyyah (unit of weight) and he weighed it for me, giving me more than its price." That means he paid Jābir (may Allah be pleased with him) more than his due.

They used to deal with money by weight not by units, although sometimes they used it by units, but mostly by weight.