Different payments by neighbors for concrete casting

Shalom.

Several tenants together built a framework of concrete casting and columns. The contractor priced each tenant’s share differently, and for the lower-floor tenant he gave a discount and charged him less for the concrete casting than he charged the upper-floor tenant. May the upper-floor tenant demand that the lower-floor tenant participate in the difference?

Shalom u’vracha.

Yes, he may demand that he participate in the difference.

In siman 158 it is explained that if one person built and the other benefits from the construction, he must pay him. (And in siman 268 the Rema distinguishes between a case where he acted on his own initiative, in which he is exempt if the other did not lose anything, and a case where he acted on behalf of them both. Now, the lower-floor tenant can certainly demand from the upper-floor tenant, since he can say that a good-quality casting was necessary so that the upper one would also be able to build in due time. But as for the upper one demanding from the lower one, at first glance the lower one has not lost anything, since he would have done this for himself in any case. It must therefore be said that in construction in shared buildings, everything that is built is considered to be done with the intention for everyone, and so is the custom, and so is the halacha.) One must examine whether each person is considered obligated to build half, or that together they are both obligated to build the entire structure. Likewise, if they are both obligated to build everything together and the contractor gave one of them a discount, one must consider whether the discount belongs to him alone or to both of them. In siman 178:1 the Shulchan Aruch states that if a tax collector forgave one of the partners his debt, the profit is shared equally, but if the tax collector said explicitly that he is forgiving him (personally), the profit belongs to him. Here, then, it is necessary to clarify what the reason for the discount is: whether it is due to some specific factor related personally to him, or whether he succeeded in lowering the price by bargaining.