What do you already have on hand? What tools and materials?
If you had a cinderblock, log, car-jack, or other heavy / sturdy item, you could use that to rest the sofa on temporarily while you work on it. Since it’s a sofa, you could even just tip the sofa on one of its sides (if there is room). Even a stack of books could work, assuming nobody sits on the sofa or uses it (which is a good idea while it’s being repaired). Either way, you probably don’t need to buy something to rest the sofa on.
In terms of re-attaching the leg to the sofa, it depends on what you are working with, whether there is a way to screw something into the leg. Imagining a wooden leg, I could imagine drilling a hole into the leg and into the sofa, then driving a wooden dowel into the leg and putting wood glue into the hole and around the dowel and then softly tapping the leg into the hole you made into the sofa - the dowel going in the hole, I mean. Sometimes screws can be driven in at angles, or you can make or use brackets that screw into both.
Honestly, desire is a justification on its own. If you have limited resources, then it’s really about whether you can afford to prioritize those desires or not. For those with enough money, desire is enough of a reason to justify a purchase.
Without enough money, the question is whether the desire outweighs other more practical ways that money could be spent. When I was depressed sometimes I would make irrational spending choices because it helped me feel good and even helped me pretend I was not as poor as I actually was, by spending the money irrationally I could create the illusion of having more money than I had. This was a dangerous game, though.