Filling in for a couple days while the department tries to rebalance itself from a loss of an employee shouldn't really be that big a deal to somebody.  It allows the company & the store to float on by for a few more days while they readjust + it gives this employee who is filling in the opportunity to work elsewhere and see if it will be a good fit.

Apparently you do not like this particular position so go tell the department manager.  Say that you were glad to help out for a couple days but honestly prefer to not continue working in this department.  They will probably tell you that they are hiring a replacement and once that person is set then you will no longer be needed.  at that point you will have made it clear that you do not wish to be that replacement and you will know that they are not expecting you to be the replacement but are trying to find somebody. i think if you really want to press the issue, your union contract should state that you do not have to work a different department if asked or told to do so.  but being helpful when in a time of crisis is a good gesture. 

i am wondering if they approached you prior to going there for a couple days and asked if you would do it or did they just all the sudden put it on your schedule. it would of been nice of them to come ask you if you are willing to do this for a while then you could say yes or no.  i presume that they didnt do so.  Go mention it to the department head then if they do nothing go tell the co-manager then tell the union steward.  Or suck it out and you will be rewarded in the end somehow b/c of your willingness to help and not freak out over a few days of working elsewhere.