What is the promise of Sydney Carton to Lucie that foreshadows a future event?A Tale of Two Cities by Charles Dickens

As one of the "hundreds of people" who come to Soho and
stop at the Manette household, Sydney Carton is a regular visitor.  As if to soften the
effrontery of Mr. Stryver's aspirations to have Lucie marry him, Carton, ironically
termed by Dickens as "The Fellow with No Delicacy" enters Lucie's room to talk with her
in Chapter 13 of Book the Second of A Tale of Two Cities.  In great
despondency, he resigns himself to his "profligate
state:

"It is
too late for [change].  I shall never be better than I am. I shall sink lower, and be
worse."

He apologizes to Miss
Manette for her compassion as he tells her that he would have been conscious of her
misery had she loved him.  And, knowing that she can have "no tenderness" for him,
Carton tells her that she has been "the last dream" of his soul.  Since he has known
her, Carton has had new ideas of "striving afresh," but he fatalistically declares that
his hopes are mere dreams.  Yet, he asks her to let him carry the knowledge that there
is something in him that Lucie can pity.

Sydney Carton then
tells Lucie his last avowal:

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"My last supplication of all is this; and with
it I will relieve you of a visitor with whom I well know you have nothing in unison, and
between whom and you there is an impassable space. It is useless to say it, I know, but
it rises out of my soul. For you, and for any dear to you, I would do
anything. If my career were of that better kind that there was any
opportunity or capacity of sacrifice in it, I would embrace any sacrifice
for you and for those dear to you. Try to hold me in your mind, at some
quiet times, as ardent and sincere in this one thing. The time will come, the time will
not be long in coming, when new ties will be formed about you—ties that will bind you
yet more tenderly and strongly to the home you so adorn—the dearest ties that will ever
grace and gladden you. Oh‚ Miss Manette, when the little picture of a happy father’s
face looks up in yours, when you see your own bright beauty springing up anew at your
feet, think now and then that there is a man who would give his life to
keep a life you love beside
you!”

Sydney's
pledge to give his life if necessary for her happiness is a foreshadowing of the final
chapters in which he sacrifices himself as Charles Darnay's double, again.  As the
sacrificial lamb, Carton goes to the guillotine in the place of Charles d'Evremonde,
nephew of the nefarious Marquis d'Evremonde.