France pulls last gold held in US for $15B gain

https://www.mining.com/france-pulls-last-gold-held-in-us-for-15b-gain/

Is anyone here actually reading the article? Yes, they really made a gain of $15B:

> But instead of refining and transporting the gold, it opted to sell the bars and purchase new bullion in Europe. […] Due to rising gold prices, the move helped the bank to generate a capital gain of 13 billion euros ($15 billion),

This doesn't make sense. If they first sold the bars held in the US, then the gold prices rose, then they bought gold in Europe, how the hell did that amount to a capital gain of $15b? How exactly do prices rising over the course of the process lead to these $15b?
Dumpling $15B on the market should lead to a drop. Anyway, the gold price is not always going up.

The claim is that rising gold prices lead to gains of $15b. As in they started with 129 tons of gold in the US, then they sold that and bought gold in Europe, and in the end, due to rising gold prices, they had 129 tons of gold in Paris plus $15b extra cash. Please explain a hypothetical course of events which makes this plausible.

Keep in mind that 129 tons of gold is worth just a bit more than $15b, so small market fluctuations on the scale of 10% isn't enough by itself.