Quote Geoff="Geoff"There used to be a clause to allow dispensation for season-ending injuries, but I'm not sure if that's the case anymore (thinking about MM's wage here). Dunno about loan players, but it would seem unfair and illogical for Rocky to count fully on both caps (ours and Cas) at the same time. Not that logic is the RFL's strong point, I admit.'"
As Magic Superbeetle alludes to the cap doesn't work in the straight forward manner your making out.
Best/Simplest (Although maybe not 100% accurate as there will be subtle small print) is that if you imagine each club can pay £150,000 per month on wages (Assuming players are paid monthly and a salary cap of 1.8m etc.) they can't spend over that each month.
Your wages have to stay Pro-rata per month under the yearly Cap amount
You can't pay £160k one month and then £140k the next month as the month you pay £160k in you are over the cap as the Full salary for those players would be 1.92m over the year(160k x 12months).
So Hampshire's wages are currently being counted on Castleford's Cap for the year as above however if he came back to Wigan it would free up space on Castleford's and count towards Wigan's from that point.
"If" Wigan are currently paying say £148k per month in Salary cap counting wages (Excluding Long service, Marquee etc. exemptions) and Hampshire was say earning £4k Per month (That's a 48k salary for the year which I am not saying he is on just using for illustration purposes) then we couldn't Bring him back unless we freed up another 2k on the cap Monthly which would mean loaning/releasing players for the period Hampshire played/stayed at Wigan.
Once got a Tutorial from DaveO on this matter and that was the best way I could understand it!