If I were doing this, I'd insert a "helper" column next to the debits
column and use the ABS() formula to find the absolute value of each
debit. I would then match the ABS() formula results to the credits on
the other worksheet.

Do you need help with the lookup portion of this as well? The MATCH()
formula might help, but your solution may be determined by what
results you need, or how you will process and use the information
subsequently.