Well good news. The correct way to treat the rent payment in my view is a reduction in purchase price or basis. This is one of those areas where there is nothing in black and white.
Per Pub 527, payments received for several /various things decrease basis. An amount you receive for granting an easement reduces the basis or cost of the property Granting the seller a rent back is similar to an easement and could be handled the same. Since this was in the sales contract and the buyer received money back at closing, I believe it is a reduction in sales price or basis.
