To combat this expansion in slavery the British Consul in Muscat continued to put pressure on sultan said to end the slave trade. in September 1822 said signed an anti-slavery treaty with the British captain Fairfax Moresby which prohibited slave transport south and east of the 'moresby line' drawn from Cape Delgado, the southern limit of the sultan's domain in africa, to Diu Head on the coast of India. (see Zanzibar and the slave trade ma, in the guidbook.)

This treaty meant that the transport of slaves from Zanzibar to the Mascarenes and India was banned, but still permitted between Zanzibar and Oman. The sultan was also banned from selling slaves to Christians, which included the French for their Indian Ocean islands. British warships gained the right to confiscate any dhows found carrying slaves in forbidden waters. Ironically, British prohibition of the slave trade to the Mascarenes only led to an increased development of the slave trade in Zanzibar itself. Sultan Said lost the revenue he would have received as duty on all slaves sold, so to make up the shortfall he encouraged the development of more clove plantations.

Meanwhile, Sultan Said continued to attempt to oust the Mazrui Sultan of Mombasa. In 1823 the sultan asked for British protection against Oman. Captain William Owen of HMS Leven saw that he could use this local dispute to Britain's advantage: he sailed to Muscat and informed Sultan Said that he intended to grant the Mazrui request for British protection unless Said agreed to end the slave trade. Said refused to do this, so Owen declared Mombasa a British Protectorate, along with the coastline from Malindi to Pangani, on condition that the Mazrui sultan agreed to abolish the slave trade. The sultan agreed, but within a few years, the Mazrui reverted to slave trading, and the British Protectorate was lifted in July 1826.