You can simply place all the branch files in a folder called 'branchfolder' and folder, drop the for next loop and use
subfield(Filename(),'.',1) as [BranchFromFilename],
from branchfolder\Sales*.xlsx (ooxml, embedded labels, table is Sheet1);
and if your branch files have the branchname/number already in a field you dont even need the subfield line!
If I right understood, you can use a map table for according branchNames.
1) create a map table:
BranchMAP: MAPPING LOAD BranchNo, Branch FROM Excel2.xls...
2) use it map for creating excel filename
for i=1 to 10 (10 - it' example)
LET vFilename = 'Sales' & ApplyMap('BranchMAP', $(i));
load Results from $(vFilename).xlsx (ooxml, embedded labels, table is Sheet1);