Is it permissible to buy a wheelchair from a street vendor at a low price, with a soldier nearby, and use it for 'Umrah?

The cheapness of a commodity does not necessarily indicate that it is stolen. However, if it is known or highly probable that the commodity is stolen, then it is not permissible to purchase it. This is because doing so involves consuming people's wealth unlawfully and cooperating in sin and aggression, in accordance with the Almighty's saying: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty). But if there is no sound strong presumption of theft, then the basic principle is that a commodity offered for sale belongs to the one in possession of it.