Matt, there's no way to do that, you need to free up some space so that the job completed successfully and could apply new retention policy (retention applies at the end of the job). What backup method do you use (forward/reverse incremental)?
FYI, there're built-in space notifications
that allow to track free space on the repository to avoid such situations, as well as Veeam ONE capacity planning capabilities.
Also, v9 will feature the new Unlimited Scale-out Backup Repository
that perfectly addresses this kind of issues.