In the most likely scenario in which Foles buys his way to free agency,he would count toward the compensatory pick formula. Assuming the Eagles don't sign pricey free agents of their own, thus offsetting the loss of Foles, they would be in line for a 2020 third-round comp pick for losing him.
Now, that we've gotten Foles' offseason scenarios out of the way, let's get back to Lammey's tweet, which mentions that the Eagles don't have enough space for a tag-and-trade scenario. At the present time, that's true. In fact, they don't have enough cap space to trade Foles in any way whatsoever, with their money allocation constructed as is.
Back in December, the NFL announced that the 2019 salary cap would be somewhere in the range of $187 million to $191.1 million. Using the $190 million projection that OverTheCap.com utilizes, the Eagles would be $15,450,726 over the cap if the new league year started today, and...
Foles were on the books at the $20,600,000 cap number he would cost if the Eagles were to exercise his 2019 option...
...and he didn't buy his way out of it.
That fact makes a trade more difficult than it already is. Why? Well, Foles cannot be traded until the new league year begins. Unfortunately, that works against the Eagles. One of the NFL's asinine rules is that players who are traded the moment the calendar flips to the new league year still have to first fit under the their original team's salary cap before they can be dealt. In other words, the Eagles would have to find alternate ways not involving Foles to shave off the aforementioned $15,450,726 they are projected to be over the cap. That would require some heavy restructuring or termination of players' contracts, with the most likely candidates being Timmy Jernigan, Rodney McLeod, and Jason Peters.
In the event the Eagles slap the franchise tag on Foles, they would have to clear even more money. Foles' number on the cap would jump from $20,600,000 to something closer to $25,000,000, which would mean the Eagles would have to clear around $20 million in cap space through cuts and restructures to get under the cap, as opposed to the projected $15,450,726 noted above.
The franchise tag also comes with another major risk, as Foles could also just quickly sign the franchise tender, which would immediately be fully guaranteed, and the Eagles would be stuck with him for around $25 million in 2019.
The bottom line: The Eagles would be crazy to use the franchise tag. This is not news.