Can you try this
returnStr = '';
if(FusionPro.Composition.isPreview == true || FusionPro.inValidation == true)
{
Rule("OnJobStart");
}
numRecsExtDF = externalDF.recordCount;
for (recordWalker=1; recordWalker <= numRecsExtDF; recordWalker++)
{
if (ReplaceSubstring(externalDF.GetFieldValue(recordWalker, 'Facility'), "&", "&") == Field("Facility"))
{
returnStr += (ReplaceSubstring(externalDF.GetFieldValue(recordWalker, 'Facility'), "&", "&");
}
}
return returnStr;