The Eastern Roman Empire was Gold vending machine, you just needed to shake it

[Aurelian]

 

[Idilinaa]

It was good policy, because they managed to pay off their enemies to last 1000 years after the fall of western rome.

They had access to vast gold mines in the mountains between the Balkans and Anatolia that they used to their advantage.

It is explained by a chronicler named John Malalas:

When the Persian emperor heard this and had been convinced by their statements, he withdrew from the agreement to make the treaty. He made his excuse the question of the gold-bearing area that had been discovered formerly in the time of the emperor Anastasios and was under Roman jurisdiction; these mountains had formerly been part of the Persian state. The gold-bearing mountains lie on the border between Roman Armenia and Persarmenia, as the experts say. These mountains produce much gold, for when rain and storms occur the soil of these mountains is washed away and pours out flakes of gold. Previously certain people leased these mountains from the Romans and Persians for 200 litrai of gold, but from the time the mountains were taken over by the most sacred Anastasios only the Romans were in receipt of the revenue that had been decreed. This was what upset negotiations over the treaty.

The mines were acquired by Anastasius I he is portrayed in the sources as an economically minded emperor who amassed a huge amount of wealth for the state.

Anastasius I Dicorus - Wikipedia

en.wikipedia.org

They maintained that control of the mines through shrewd diplomacy tactics. The mines gave them their own sources, Gold was also sourced from Nubia and Ethiopia apparently, although i am not really sure since i have only seen it mentioned in secondary sources.