So does that mean once we do that we are spent up to the cap? And then we can do the MLE and LLE? I also don't understand the "cheaper to keep Jackson" comment.
Once we're up to the cap (or so close to it that signing someone would put us over), we'll only be allowed to sign people using exceptions. This would mean Bird Rights if we had any left this offseason (we don't), contracts for 1st round picks (which is irrelevant to us right now - their cap holds are already factored into our cap calculations. It only really applies to teams that are already over the cap), mid-level/bi-annual/room exceptions (we only have the room exception because we used our cap space to sign someone, which is good for ~$4 million), and minimum salary exceptions (any team, no matter their cap situation, can sign as many minimum salary players as they want, provided they don't violate roster limits). We'll be able to sign one FA with the room exception (it can be split, but that makes no sense given how close it is to the vet minimum), and then as many as we want to minimum contracts.
As for Jackson, it's just a peculiarity of his contract - his guaranteed money plus the empty roster spot hold ($650k+$815k=$1.465 million) is higher than his salary this year ($1.385 million), meaning that cutting him would actually reduce the cap space we had available. Trading him for no returning salary, on the other hand, would not have such an effect.