Client A wants to make an offer for a software consultant position but
they need to assure that I will accept it
Consider their position; it always looks bad when you make a formal offer to a consultant, set up all the paperwork and planning, and then they turn around and go "changed my mind". If this is the situation, then you're basically getting the offer already, they just don't want to do a lot of work getting sign-offs if it's going to be wasted.
In which case, the principled and simple thing to do is to let them know that you can't make the decision immediately. For reasons, you can say you want to discuss it with your wife/family/"life coach"/wise old mentor who lives on top of a mountain with very poor cell phone reception. Don't lie ("killing off Grandma" always gets you in trouble), but if you need to rebrand one of your mates as your life coach for half-an-hour, that's okay.
If they really do need an answer right away, then they really only have two options; pay you whatever you ask for, or find someone else who will start right away. In the real world, they'd merely like an answer right away, but can at least wait till they find your competitor.
At the same time, it's not unethical to let the other company know that you've been approached with an alternative offer that would need you to start immediately (and thus be paying you immediately), and that you need to fully understand where you are in their process, so that you can make a sensible decision whether to take the certain job or hold out for their potential job offer.
As far as negotiating goes, a couple of things drawn shamelessly from the book "Getting To Yes":
- What is your Best Alternative if you try to negotiate, Job A falls through, and then you don't get Job B? (e.g. "Tap into your Savings" > "Parent's Couch" > "Cardboard Box under a Bridge")
- What is their Best Alternative if they don't hire you? (e.g. are there many other equivalent role fillers in your area?)
- What do you actually want? It's amazing how poorly most job applicants can answer this question beyond "more money than the original offer". If you can say "I want X, Y, Z, and the option to get A with the next 6 months", you'll be in a much better situation.
Also, note that just because they ask for a commitment, nothing about that is meaningful till you sign the contract. Until you sign and return, you retain the option to change your mind with nothing worse than bad feelings on the recruiter's part.