Easy to read is:
Manager: Ms. A / Mr. A is leaving but we couldn't find a replacement yet. Can you temporarily handle her/his job until we found a replacement?

You know very well if you can handle it good, the job's sticking with you for long and they will not hire another person to replace A.

Is there a way to decline such requests?

Cannot say no. I tried saying I cannot but the boss said need to until a replacement is really found. For now, they are actively finding for that new position but not many meet their criteria. Even if got also, they are asking for salary that beyond the company can offer for that post.

Rumours are abound that this head count might be removed and be absorbed by another dept as that dept also needs an emergency headcount. I told my boss I can't juggle all these work but my boss said the top mgt has considered putting extra RM300 for me every month to handle this job. I feel so cheated.