Paul Williams, 23, from Cardiff , originally asked partner Seren Pedler to marry him moments after being told he had months to live at Velindre Cancer Centre.
When volunteer Karen Hobbs, from charity Wish for a Wedding asked him if he had any regrets, he spoke of his desire to make a more romantic proposal.
The charity set up the moment to remember for the couple at the home of the bride-to-be’s father Mike Pedler and stepmother Hayley Pedler, in Blackwood, Caerphilly .
With a diamond ring in hand Mr Williams, who was diagnosed with bone cancer, also known as Ewing Sarcoma last October, sprung his surprise on his 21-year-old bride-to-be.
The proposal appeared on the front of the house opposite after Mr Williams got down on one knee on the balcony at the home of his future in-laws.
Ms Hobbs said Mr Williams, who has been lapsing in and out of consciousness with his illness, came to life when told of how the charity had set up the plan.
She said: “He was lying on the sofa lifeless and then all of a sudden there was just this big smile and he said, ‘Oh my God’.
“It’s as if he’s going to give her what she deserves.”
Mr Williams had hoped to make his proposal on Saturday evening at Côte’s Restaurant, in Cardiff Bay, with the laser writing appearing on the iconic Coal Exchange building.
But this had to be cancelled when he was rushed into hospital.
The couple are expected to tie the knot on Friday after Mr Williams’ cousin Louise Walpole spearheaded a social media fundraising drive to pay for the nuptials.
The Cardiff couple have a one-year-old daughter Amelia-Mae, who turns two in September and will be a flower girl at the wedding.
They also plan to celebrate her second birthday at the wedding.
Mr Williams has written birthday cards for his daughter so she has a greeting from him every year until she is 21.
Donors have also given charms that she can add to a bracelet as a gift from her father until she is 21.
One of the heart-shaped charms contains a picture of Mr Williams with his young daughter.
Ms Hobbs said: “She’ll always have that on her bracelet to remember her daddy by.”
Another donor has offered to buy a specially-built teddy for Amelia with recordings of her father’s voice reading bedtime stories.
And Barry Round Table has organised a trip to Drayton Manor Theme Park, in Staffordshire, for 15 members of Mr Williams’ family.
Ms Hobbs added: “I’ve organised a photographer to go with them for the day – so Amelia can look back at photographs of Paul and his family.”
Mrs Walpole, 31, has organised donations to pay for the wedding ceremony at the Church of the Resurrection, in Ely, Cardiff.
Ms Hobbs said the diamond ring was donated by another charity and Miss Pedler, who already has an engagement ring, will be able to wear it around her neck for the rest of her life.
A GoFundMe page set up for the couple has already raised more than £1,600.
Miss Pedler’s step-mother Hayley, who has also helped with the fundraising effort, said another of Mr Williams’ final wishes was to take Amelia to Disneyland.
Ms Hobbs said the fundraising drive has so far prioritised paying for the costs of Mr Williams’ funeral.
To donate vist www.gofundme.com