Preparing return for elderly client who received principal and interest from an installment sale, seller financed, of one of his rental properties. We pick up all of the applicable principal, and 100% of the interest. We have the name and address of course of the individual who paid it, but he apparently does not have the social security number. Would this prevent e-filing? Could I efile it and disable error checking? Client is elderly and not well, and I'm not sure if he's going to be able to obtain the social security number, and some people don't want to give it anyways, i.e. the identity theft paranoid crowd. Thanks for help.