You could try something like this (without having your tables, I couldn't test that this actually works):

SELECT POPRCTNM,

STUFF((SELECT ','+ CAST(ACTINDX AS varchar)

FROM POP30390 a

WHERE a.POPRCTNM = d1.POPRCTNM

GROUP BY a.ACTINDX

FOR XML PATH(''), TYPE).value('.','VARCHAR(max)'), 1, 1, '')

FROM POP30390 d1

But if your goal is to search for a receipt containing a certain number, you should be able to write this as a query without using the FOR XML hack. What conditions are you looking for that would identify a user-entry error?