Since the Russians are unlikely to accept anything less than hard currency from the Egyptians for payment it begs the question of where Egypt would get the money for such a purchase, the price of which would undoutedly be very high since the S-400 is presumed to be the best air defense system available.
Well, the logical answer is that they would use US military aid (the Foreign Military Financing Funds that the Egyptians still get every year as an incentive for signing the Camp David Accords in 1979). But, would the US sign on to Egyptian purchases of Russian weaponry, given that there is some built in expectation that the Egyptians would use that money to purchase US defense material and at the very least would not buy material from the Russians?
A possibility I never considered (although now it seems entirely logical) is that the Americans would actually support such an Egyptian-Russian transaction on the premise that the Egyptians would give the Americans a few of the SAMs so they could use them in field tests (as opposed to having to resort to simulations and other computer generated tests. This would be especially beneficial to the US given that many of the countries that have bought these Russian weapons are those likely to be engaged in confrontation with the US in the future.
I would also add, incidentally, that on 6 July Mubarak promoted, with much pomp in the official papers, a new class of officers in the air defense branch of the military (Egypt has this separately organized). Another question worth asking is who is brokering the Russian deal, should it go through.