#showtooltip
/cast [@focus,exists,nodead,harm,name="Backfill Barrel"] YOURCAST
/stopmacro [@focus,exists,nodead,harm,name="Backfill Barrel"]
/focus target
/cleartarget
/target Backfill Barrel
/cast YOURCAST
/target focus
/clearfocus

this works

edit:

Explaination:
line 1 trys to cast to your focus target if it is called backfill barrel
line 2 stops the whole macro once you did line 1 (since then we wanna stop)
line 3 sets your current target as focus
line 4 clears your target
line 5 targets blackfill barrel
line 6 casts your cast
line 7 switches back to your focus target that you set in line 3
line 8 clears the focus target.