Arsenal will never let him go, especially not in the middle of the season. He and Leno are basically the only reasons Arsenal isn't in the relegation zone this season...and even though he's class, he's also 31, enough with these 30+ purchases, not to mention his speed is a big part of his game, he relies on it a lot besides being a deadly finisher. But I don't see him staying on top or even current Barca level for more than 2 seasons anymore, tops.
He could be a stop gap if we decide to go all in for someone like Mbappe in 1-2 years, if he doesn't sign an extension with PSG soon (current contract is until 2022, I think), but otherwise there's no point in bringing Auba, especially since he's the saviour for Arsenal right now and I don't see them letting go of him for less than an obscene amount of money, which as mentioned, would be another big and stupid investment into a 31-year old. If we go for someone this summer or even in January, I'd rather see us take a punt on Werner, who seems destined to leave Leipzig this summer, or Lautaro, if he's willing to leave Inter this soon.