He has a whopping 3 games in the Bundesliga a couple of seasons ago and was highly rated with a few games in both the PL and Europe when he signed his last contract so it's not really that big a surprise.
It's more due to them handing bloated contracts to the likes of Özil, Willian, Luiz and so on and paying massive fees for Pepe over even worse players like Torreira, Saliba and Mkhitaryan who are not even playing for them atm.
£20k to a young talent I'm sure they can handle.