Hmmmm, Swolen bush could be nylon (hydrophilic) it tends to absorb liquids and expand, you could try leaving it in the airing cupboard for a week, not sure if that would work though TBH….
Alternatively you could get a replacement turned up in Delrin maybe or better yet see if it has the same dims as a standard part(s) from IGUS or similar in something intended for shitty environments, or contact the supplier and ask about a replacement bush…
Of course by the time you’ve gone through all of that faff it might just be easier to to buy a different post…
The trouble with boots/old bits of inner tube is for all the crap they keep out they still run the risk of allowing a small quantity in and hold it next to seals/bushings making the situation potentially worse than if it was open to the elements….
Might I ask did you wash down/clean the post and then leave wet at all, with the boot in place? you might have inadvertanatly soaked/trapped moisture next to the bush in question and actually caused it to swell, well intentioned maintenance could in this instance lead to damage/failure…