This Apollo thing would be some of the Glazers being bought out of the club. Apollo would get their shares and they would walk off with the money.
Joel and Avram would be left in charge of the club.
No money would be invested into Manchester united by Apollo and no money by Joel and Avram because they wouldn't be getting any.
When Malcolm Glazer bought Manchester united he did it almost entirely with money he borrowed at incredibly high interest rates. Think almost payday loan levels. This was just to get the deal done. This was then refinanced at huge cost to the club, not the Glazers, into lower interest loans which, even to this day have not been paid off at all. Instead they have used United revenues just to pay the interest.
Does this help you understand my concerns now? They are not seeking money for the stadium or to buy players. 4 out of 6 Glazers want out of United and to use the money elsewhere. No equity firm worth its salt would do this deal and not want huge control in the company. However, Apollo are allegedly quite a shady outfit and so may just want to do a deal like this one. But they will be doing it in the hope that they will profit from a full future sale.