True ... And maybe our type of dogs are different ... But Ben had no concept of leave or wait for food. Literally in less than a minute he knew that "trust!" meant to not touch the titbits between his paws ... And I use the "leave" command for times such as when Gucci has disappeared in his mouth when trying to sneak ben's bone! All dogs are different though. Tai was a nightmare when he first came to us. You almost lost a finger if you gave him a titbits, but again he learned that my hand did not open up to reveal the biscuit until he had backed off and was sitting down. He was not as smart as Ben but very quickly he learned to "trust" and to "leave".

Please don't listen t the suggestions of "letting the cat sort it out" - the poor animal is almost defenceless and would be an absolute sitter for the ghastly brute you are trying to train into a reasonable companion. Personally I am not so tunnelled in my vision of dogs (many see them as some sort of fallen gods) and would get rid of the damn thing back to the shelter or wherever so that it can go to a home where interaction with cats or similar small furries would be severely limited.
Ref the comment about dogs being put off by cats arching their back etc, they have obviously never come across lurchers or greyhounds, virtually all of which would kill a cat that stood it's ground. I have three lurchers and a terrier that live with six cats in perfect harmony, but even they (even the small fell terrier) would very swiftly kill a cat they came across in the fields or woods if I wasn't there to bellow "leave" and avert the potential catastrophe.
If you do wish to persist, try following tawneywolf's advice or get a very hands on trainer for it.

The cat was adopted , it might have been declawed before it was adopted

Its not a good idea to let a cat sort out a dog.

To the OP ,

Tawneywolf has given good advice.

I have adopted or fostered animals, cats , dogs, rabbits , guinea pigs , birds and successfully got them to accept each other. The dogs have reacted very excitedly , barking , pulling and whining and I worked on them , But , one time I took home a dog , and as soon as she saw my cat , she had such intent and determination that I just knew it wouldn't work and that she would very probably really hurt my cat so i decided not to foster her.

I'm sorry you are in this position. You have a very difficult choice to make.

Do not believe these people. My rescue dog ...same size as yours.... went for the cat as soon as she arrived. I rather naively thought I could train her and I took her to the cat who was in a safe place and gave the Leave it command. She turned away. this went on for months, with the cat living in the hay barn.
One day however with dog on lead and cat behind wooden fence she saw her chance, grabbed the cat and killed it.