I assume you mean the tank pump. I changed one on a '85 740 once. It was quite a challenge. I think the operation may be similar on your car. If so, I am glad to offer this idea and hope it helps you. If, like me, you are doing your own work to save repair costs on a car that has depreciated down to very little, the expense of a one-trick Volvo tool defeats the purpose. Although, it would have been really nice to have one when I did this.
On the 740 there is a small rectangular panel in the trunk that provides access to the tank pump assembly. The access hole underneath the panel is barely any larger than the nut and only an inch or so above the closest point of the nut. You can touch the nut easily but it is impossible to position your hand(s) so that you can get any leverage on the it. The hole would need to be much larger to allow that. The nut may have been installed by hand at the factory but over the years it ends up very tight (shrinkage, etc).
All I could think of to do was to use a hammer and a cheap blade screwdriver with a long shank and very carefully pound the end of the screwdriver as you would a chisel. What you are trying to do, though, is to get the nut to move, rather than to actually chisel into the nut. There are stubby flanges all around the circumference of the nut. I placed the tip of the blade screwdriver in the corner where the very short flanges meet the rest of the body of the nut. Then, while keeping the tip there, tried to position the screwdriver as close to a tangent to the circle of the nut as possible. Then I started tapping away, gradually increasing the force of my hammer-strikes as my frustration grew. As it turned out, the nut was able to sustain a pretty good whack without cracking. Ideally you would want to work your way around the circle, tapping on each successive flange in case the build-up between the threads of the nut and what it is threaded onto has become harder at some position around the circle than at any other position. But the position of the nut in relation to the access hole only allows you to position the screwdriver nearest to a tangent to the circle (as I recall) at one point towards the front of the car and one towards the rear, or, perhaps it is better to say at the highest point on the nut and the lowest. I should probably mention that I was actually sitting in the trunk with an emergency light in order to do this. It was maddening but eventually I was able to dislodge the nut this way so that I could unscrew it the rest of the way by hand. What slowed me down the most were two things: 1) All the time insisting to myself that there had to be a better way and trying to figure out what it was, and 2) not striking hard enough with the hammer out of fear of damaging the nut.
Good luck, you can do it!