Ok, try this (it works in my copy of your report...)
1. Create a formula I'll call Total Credit Weeks Calc and put it somewhere in the detail section. It looks like this:
Shared NumberVar totalCredit;
if OnFirstRecord then totalCredit := 0;
if OnLastRecord or GroupName ({CHECK_MSTR.PAY_TO}) <> next({CHECK_MSTR.PAY_TO}) then
totalCredit := totalCredit + {#RTotal1};
" "
The final " " is so that it doesn't display anything.
2. Create another formula I'll call Total Credit Weeks Display and use it for your grand total. It looks like this:
Shared NumberVar totalCredit
-Dell