Disable Unique Transaction ID validation
in WHMCS v6.x was added transaction validation in admin area -> Invoice -> Add Payment
and some time we have same transaction ID from bank transfer or Credit Card gateway manual transactions.
this make the work in billing to less efficient and more cumbersome.
Please add some option in Setup -> General Settings -> Invoices that say "Disable Transaction ID validation"
selection of this option will disable this Transaction ID validation and allow to use it as it's was before this new validation.