When in Split recently, I saw some (frankly rubbish/broken) bikes parked in certain spots and downloaded the app to use one.
The minimum to put into the account was 10 euros and the ride cost less than 1 euro.
Aside from not recommending it if you value things like working brakes, I see from their T&Cs they will not return any unused money.
https://www.nextbike.hr/media/General_Terms_Croatia_EN.pdf
7) The user can not transfer nextbike credit to another account, nor can nextbike credit be used for any other purpose other than lease rental, activation fees or tariffs and the order of customer cards.
8) Nextbike CROATIA is not obliged to refund nextbike credit because this applies to the mandatory annual fee to use the system.