Though most of the Brown brothers trading activities were directed to the Caribbean, they also traded with mainland colonies. A voyage to Virginia by the sloop Four Brothers in 1763 touched off a correspondence between the Browns and Carter Braxton, a Virginia merchant and future signer of the Declaration of Independence, who proposed working together to mount a slaving voyage to Africa. "I am told there is a great Traid carried on from Rhode Island to Guinea for Negroes," Braxton wrote in this letter of February 1, 1763, "and I should be glad to enter into Partnership with some Gentlemen for a Voyage or two and have [the Negroes] sent here where I believe they sell as well as any where." The Browns initially seemed receptive to the offer, but ultimately decided to mount an African venture independently.

Having had no reply from the Brown brothers to his letter of February 1, 1763, proposing a partnership in an African slaving voyage, Carter Braxton wrote again on June 1, 1763. Braxton does not mention the slave trade in this letter, but instead emphasizes the possibilities of a bilateral trade between Virginia and Rhode Island, especially following the recent repeal of a duty on rum by the Virginia legislature.

Letter, dated October 16, 1763, from Carter Braxton of Virginia to Nicholas Brown and Company, continuing discussion of a possible joint African slaving venture. "I shall be very glad to be concerned in the Affrican Trade and will be a fourth of the Voyage if you Choose it," Braxton writes. The balance of the letter offers information on the local market for slaves in Virginia. "Gold Coast Slaves are Esteemed the most Valuable + Sell best," Braxton writes, adding: The Prices of Negroes keep up amasingly." For reasons that are unclear, the Browns did not pursue the partnership, and instead launched the Sally independently.

Letter, dated September 5, 1763, from Nicholas Brown and Company to Carter Braxton, a Virginia merchant, replying to Braxton's proposal that they join together to mount an African slaving voyage. "You Mention of being Concerned in the Guine Trade and that the Vessels Return with the negrows to your place," the Browns write. "[A]s we Shall be Largely Concerned in Navigation this Fall which will bring millo. [molasses] in the Spring and we Living in a place wair we Can procure a Large Quantity of Rum Distilled Amediately, its Very Likely if it's Agreeable to you to be Concerned that we May Fitt a proper Vessill for Guiney in the Spring..." In the event, the Browns did send the Sally to Africa in 1764, but they did so without Braxton.