It's enriching yourself if you use OPM to pay your own companies for goods, services and rents that you can purchase from others. By definition, he is economically better off if he rents space from himself that wasn't currently being used by others. Further, he is doing a disservice to donors by renting the most expensive space he can find versus quality space that is far cheaper.
I'm not saying it's illegal, but it is certainly unseemly and an unwise of donations.
I know you're not saying it's illegal, but I'm saying that assuming everything is on the up and up, (meaning he didn't overcharge his campaign or anything like that) it doesn't matter if he utilized his assets or someone else's, it's all the same, and really even could be smarter to use his own because that lowers transaction costs and simplifies a lot (i.e. lots of flexibility and you don't have to worry about dealing with dickhead landlords)
It's not enriching yourself if OPM was going to have to pay for the same things at the same prices and his companies were going to make the same sales/perform the same services for someone else at the same prices
I said, "in an economic sense" which does not mean economically. From an economic standpoint he is no better off because the space would've been rented anyway. Economically he is better off because he has income coming in, but unless he's charging more than market, it's not more income than would've been coming in anyway, thus he is no better off "in an economic sense"....that's a bit nuance I know, but there is a difference.
Correct me if I'm wrong, but you don't know what Trump is charging his campaign or how expensive his office space is (although i'm sure it's not cheap). Also, what benefits are associated with having offices in Manhattan (I assume that's where his offices are)? I mean, he was probably going to have offices there anyway because he's that kind of guy. You can definitely argue that having an office in Manhattan is frivolous, but there is obviously an argument that it has value and unless you're an expert on such thing, you kind of have to give him/them the benefit of the doubt on that one.