This is on purpose, explanation: When you try to put the box down, it gets blocked, which rewinds to the initial input and changes it so the box doesnt enter the portal. Now the next time the portal activates it must block that past activation to maintain timeline consistency. So the "debt" is not an item to send back, but the obligation to block that box.
I think you solved it in a more complex way, all you should need is to (ROT 13) fraq n cynlre gb gur cnfg, abg creznaragyl fjnccvat vg jvgu n obk (ROT 13 end).
Level 5 is a bit much of a spike yeah, swapped it with 6 which should hopefully help. I do indeed have some more ideas for this so maybe I'll make something larger of it eventually.
