Part of the "problem" with the Nazi loot was that a fair amount of it was siphoned off in various ways. Goering's art collection is only one example, but agencies within the shambolic structure that was the Nazi government would siphon off chunks for their own special purposes, such as the SS would do. There were sticky fingers both private and governmental all the way to the (central) bank. Another issue was that while the non-monetary loot from German and Austrian Jews confiscated between 1933 and 1939 was available for use, once the war started it was difficult to turn it in to cash. While you could ship a stolen art work from occupied Europe to the USA or South America, this would require a complex route through neutrals using neutral shipping. Furthermore the market for obviously stolen works without a provenance would be limited, and things would be sold at reduced prices.(1) In terms of gold, the problem becomes how to use it. As of 1939 Germany was basically cut off from the international banking system and interbank transfers. This system allowed gold to stay where it was, or be transferred to third party repositories, while the ownership/credits transferred. Some countries might accept "credit transfers" in lieu of actual gold deliveries, but I suspect that more neutral neutrals like Turkey and Portugal would expect physical payment, and even friendly neutrals like Spain would expect gold or hard currency (not Reichsmarks). In the case of Spain, they can use Reichsmarks or swaps to buy German goods, but a lot of their essentials like oil and food come from places that want hard currency or valuta and won't accept Pesatas or Reichsmarks so credit swaps have limited utility.
Being cut off from the international banking system was a big part of Japan's problem, and is a cause of difficulties today for Iran as an example. Before all the electronic commerce, internet, etc, the dependence on the system for transfers and payments was less restrictive, but still vital for major transactions and intergovernmental dealings.
(1) The continuing issue over stolen art works centers around the issue of provenance - the "history" of who owned the art work when and the sales/transfers. Prior to the war, the Nazis worked around this by forcing sales at fractions of value. Later on these sales were justified as legitimate as the new owners would claim the "forced" sale had been because the owner was desperate for cash, not because some black or brown shirted thug held a pistol to his head. Some were taken as "tax payments" since Jews were not allowed to leave Germany (later Austria) with more than a pittance in cash. Of course once the war started, Jewish property was simply taken with no pretenses down to the dental gold in their teeth after gassing. Art works, and things like houses and cars, were then frequently gifted to Nazi party folk or sold via art dealers creating a chain of "legitimate" provenance. After the war, and until this day, the people and institutions in possession of this stuff use the paperwork to show they legitimately acquired it. Often they wave the bill of "forced" sale to show that the chain of acquisition is legitimate. A significant amount of art in museums in Germany and Austria was acquired this way, and these institutions fight tooth and nail to keep it.
Being cut off from the international banking system was a big part of Japan's problem, and is a cause of difficulties today for Iran as an example. Before all the electronic commerce, internet, etc, the dependence on the system for transfers and payments was less restrictive, but still vital for major transactions and intergovernmental dealings.
(1) The continuing issue over stolen art works centers around the issue of provenance - the "history" of who owned the art work when and the sales/transfers. Prior to the war, the Nazis worked around this by forcing sales at fractions of value. Later on these sales were justified as legitimate as the new owners would claim the "forced" sale had been because the owner was desperate for cash, not because some black or brown shirted thug held a pistol to his head. Some were taken as "tax payments" since Jews were not allowed to leave Germany (later Austria) with more than a pittance in cash. Of course once the war started, Jewish property was simply taken with no pretenses down to the dental gold in their teeth after gassing. Art works, and things like houses and cars, were then frequently gifted to Nazi party folk or sold via art dealers creating a chain of "legitimate" provenance. After the war, and until this day, the people and institutions in possession of this stuff use the paperwork to show they legitimately acquired it. Often they wave the bill of "forced" sale to show that the chain of acquisition is legitimate. A significant amount of art in museums in Germany and Austria was acquired this way, and these institutions fight tooth and nail to keep it.