I can imagine paying them once, and even a very large sum, because then they become dependent on your goods and commerce becoming deeply ingrained in your markets, but paying them twice is a mistake and three times is Art of the Deal (idiocy).

In the Principate, the first ~2-300 years of the Empire, this was actually a common practice, though not as large lump sums. Instead, Roman-aligned tribes would receive annual ‘subsidies’ in exchange for remaining Roman allies/buffer states against further afield barbarians. Such allied tribes would also often receive trading rights in nearby Roman cities in addition - for exactly the reasons you mentioned. It was a legitimately good way to shore up pro-Roman elites in buffer states and ensure they had enough force to be effective buffer states.

Funny enough, this would sometimes be negotiated in the aftermath of a tribe’s defeat by Roman forces. Gotta show ‘em who’s boss!

Unfortunately, while this practice was effective when the Germanic tribes were not being pushed out of their lands in civilization-level mass migrations, it was distinctly not effective afterwards. And of course, as a pre-established practice, it was so easy to fall back on, even in circumstances far divorced from what subsidies were supposed to achieve…