Here's a more authoritative source:
During the pre-Revolutionary years, Washington's views toward slavery were [as far as the record reveals] conventional, reflecting those of a typical Virginia planter of his time. He undoubtedly shared the "engrained sense of racial superiority" so common among white Virginians and did not emotionally identify with the slaves' plight. There is an extant letter from Washington [1766] that leaves a flavor of the nature of the institution and his rather routine acceptance of it.
"Sir: With this letter comes a Negro (Tom) which I beg the favour of you to sell, in any of the Islands you may go to, for whatever he will fetch, and bring me in return for him: one hhd of best molasses, one of best Rum, one barrel of Lymes if good and cheap, … and the residue, much or little in good ole spirits…That this Fellow is both a rogue and a Runaway…I shall not pretend to deny. But . . . he is exceedingly healthy, strong and good at the Hoe… which gives me reason to hope he may, with your good management sell well (if kept clean and trim'd up a little when offered for sale… [I] must beg the favor of you (lest he should attempt his escape) to keep him hand-cuffed till you get to Sea."
Fortunately, this is only transaction of this kind to be recorded in GW's correspondence, and there are many other later entries and incidents that reveal a more humane and caring master. The story is complicated because GW's views about MV and his slaves was an uneasy mixture of commercial and paternalistic attitudes. These aspects were often in conflict with one another and led to inconsistent action. Indeed, Washington's erratic mixture of sternness and indulgence inevitably created a certain amount of chaos in plantation management.
The middle paragraph is a copy of a letter sent by Washington where he trades a slave that he owns for a hogshead (a cask or barrel) of molasses (a type of syrup), another of rum, a barrel of limes (spelled Lymes), and the residue in any kind of spirits (liquor). Presumably this letter is the source of the claim that he traded a slave for molasses. It's not evident why they are saying a black woman, as this is a man.
In the link, the overall work is credited as "The Only Unavoidable Subject of Regret": George Washington and Slavery By Peter R. Henriques. Note that the overall work is considerably longer than these three paragraphs. They just seemed a nice encapsulation of this particular story and a bit of context that seemed to provide useful context for how this is reported by historians.
An alternative source for the same event. The letter is quoted slightly differently but the basics are the same.
The book George Washington: A Life by Willard Sterne Randall also references this story (p. 207).
The Peniel E. Joseph book Dark Days, Bright Nights uses the same "black woman for a barrel of molasses" formulation but doesn't cite proof. Also it's unclear who is actually saying it (on page 226). Someone named Carmichael who is presumably identified more fully earlier in the book.
