The problem with the rune is that you can't easily see how much gold you have, or rather with how much gold you can work with. This is because the part of the gold from the rune is shown separately and therefore you need to add the debt and the owned gold together to get your actual amount of gold. This is not a huge problem because you just need to add 2 numbers together but it's still annoying and it takes you more time to do that than to just simply look at the number of gold you have. Also the gold amount shown in hud corner (below your items, next to your spells, recall button etc.) is not showing any debt information at all and you need to open the shop and calculate to see how much gold you can have to buy an item.
There is however a simple solution for this, next to the current gold amount show the amount of gold available when combined with the gold you can take from the debt. That's the simplest solution, the only problem is the gold shown in the hud corner because it won't be showing the combined gold, but this problem already exist anyway so adding this to the shop interface won't make things worse, only better.
Another and maybe better but not simpler solution which can affect balance is to rework the rune so it doesn't have the debt fee. Then you can throw away the debt mechanic and just make it a "Free gold" rune. It will simply give you bonus gold at the times the debt rune has been giving you allowance. With this you don't need to show separate gold values and debt values, and there won't be a problem with the gold in hud like there is now, because it will simply add to the current gold and you will just have one gold value showing all the information, no more opening the shop window and calculating to see with how much gold you can work with. The only problem with this approach is that it affects the balance of the rune because you won't have to pay the debt fee anymore to take extra gold. But I think you can balance this so it gives you less gold. Also the debt mechanic is pretty ambiguous in itself, because it's not very clear when will you exactly pay the fee? Will it charge again if you are already in debt? Changing this to be just bonus gold instead of the complicated and hard to play with debt mechanic would make the rune much simpler to understand and use.